Express 5:50 Plus or 5:25 Plus

I am about to pull the trigger.
Can anyone tell me what the differences are? Other than wattage?
What kind of tone characteristics? I am gonna have to travel a long way to try em out so I would like to have some input and advice before I go.
Thanks,
Chris

Well I aint gonna be of any damned help because I have never played a 5:25+ OR a 5:50+.
BUT...I do have a 5:25 and...from my dead cold hands is how someone will take this thing from me.
I'm an old Marshall guy. Been playing an 18/36W EL84 Marshall clone since about 2006, got some other amps I had been GASsing for for a bunch of time (a Fender HRDlxe, A Marshall DSL100, my first Mesa ever-a Stiletto Ace head, and a MAZ18) which were all great.
I gig a lot. I'm 50 :-(
I like 1-12 combos these days when they aren't too heavy. The MAZ was cool. The HRD was good but I only used it's clean channel goosed with pedals for my od tones...the DSL is just a joy to play on the big outdoor festival stages...BUT...the only ones I kept were the DSL (probably had it for 5 months now?) and the 5:25 I got around 3-1/2 months ago or so.
My 18/36 out-clean-headroomed the HRD and MAZ. It has the best Marshall crunch at decent volumes of any amp I've ever owned.
UNTIL the 5:25 :-O
Only 2 EL84s but as much (more?) headroom than the MAZ, HRD, or almost even my build (18/36). It's channels are both useable. I even use the dirtiest mode of channel 2 sometimes and I'm not a real high gain guy.
It's incredibly lightweight. Deceptively punchy. SO versatile. Even the reverb sounds good.
The non-PLUS doesn't have the mesa 5-band eq. I sure don't need or even want it. I don't like scooped. I don't even keep an eq on my board anymore. I have a GREAT clean boost (Dingotone HZD) that gets used a lot and if I did NEED the mesa graph I could buy the recent 5-band eq unit they released...or dust-off the old MXR 10-band and put either in the effects loop.

The non-PLUS has a "CONTOUR" knob that "is" the signature MESA scooped mids 5-band eq setting. It is footswitchable and variable but on my 5:25 for some reason the CONTOUR circuit doesn't even work. I don't really care. It would surely be handy as a boost when using the 5:25 combo by itself without my pedalboard but...oh well. Maybe one day I'll get it working and I'll go OMG...I WISH I haddda had this thing working a long time ago!!

Now to step down off of my 5:25 soapbox and give you the reply I meant to in the first place.
The difference between the 5:25 and 5:50, besides the wattage is that the 5:50 uses 6L6 powertubes where the 5:25 uses EL84/6BQ5s.
That means that the 5:50 SHOULD have more of the MESA signature tones. There should be some sparkly fender cleans in it. Bottom end should stay a lot tighter. I assume they will be extremely (too) cold biased whereas the cathode biased 84s in the 5:25 are running pretty hot. ESPECIALLY if theyre making 25watts :-O theyre using their incredible magical DYNAwatt tekmology to pull that off. The 5:50 has a third power setting. 5w, 25, and 50. The 5:25 has only 5W and 30W. Yes I said 30. I know right?
In single ended "class A" operation it's pretty hard to beat the snot a single EL84 makes so again, depending on what you PREFER, in 5W the 5:25 KICKS some SERIOUS A$$.

If you're old like me you will LOVE how lightweight the 1-12 5:25 is. I'd guess that the 5:50 is probably a bit of a beast to pack around.

To me the 5:25 is loud enough to play on the big outdoor stages. Hey, on those stages your guitar is in the wedges and/or ears anyway. As much of a treat it is to be able to pull out a 100W and crank it into a 4-12 bottom cab on the big stages...well it isn't often that I get to crank the 5:25 so its usually a toss-up for me.
My point is that I just couldn't IMAGINE ever USING all the 6L6 power in a 5:50.

I have never even tried my 5:25 into a 2-12 or 4-12 or 2x 4-12 closed back cabs. We're in the studio doing some recording right now. That is as much of a treat as the big stages cause you get to bring-out some gems from deep in the arsenal. All the priceless or weird artifacts from the collection get to stretch their cob-webby legs when we record.
Welllll.... I'm sad to report that I used the DSL for a couple of scratch tracks and it's been the 5:25 for EVERYTHING ever since.
This stupid thing just keeps blowing mine and everyone's mind who hears it.

So..the differences, besides wattage are gonna be...EL84s vs 6L6s. The wattage...oopsies. But I mean the extra wattage selection of the 5:50 and how the 5W mode is gonna sound with an EL84 vs a 6L6...realistically I don't even use the 5W mode even in the studio :-(
the weight is gonna be a pretty big difference if that matters.
That's probably about it.

I'd go with the 5:25 probably. I would pick the 5:50 if I was finding that the 5:25 was running out of headroom...like the HRD and MAZ DID!!!!! but it isn't so...
Especially if there is a big $ difference..go with the 25.
If the money was the same for either I might take the 50 but like I said...aint never even tried one. If you can, TRY both. If you can't...pfff..either is awesome.

If you take both amps and run them through the same cab, the differences will be minimal. Mesas don't rely on power tube distortion for their signature sounds. A clean 6L6 is going to sound very much like a clean EL84 when they both have the exact same preamp in front of them.

That said, the biggest difference is going to be headroom, as in how loud you can play without the power tubes breaking up. The larger 6L6 tubes have more headroom. The new multi-watt feature of the Plus models is pretty awesome, though. I have a 5:50+, but I always run the channels in the middle 25 watt position. For some reason, I just prefer the sweetness that position provides. I have also been enjoying the Burn mode of the 2nd channel in 5 watts. It gets this nasty fuzz tone that I'm having a lot of fun with.

Now, my first Mesa was the original Express 5:25. Mine was the little 10" combo. I really regret selling that thing. It had a cool, unique sound when running through the little speaker, but when plugged into a larger cab, it really filled the sound out. I actually compared it against my dad's Mark IV one time, running them both through the same cab for a fair comparison, and I totally preferred my "cheap" 5:25.

I want to reiterate that the size of the cabinet makes a HUGE impact on the tone. If you only compare the 5:25+ combo to the 5:50+ combo, the 5:50+ will sound fuller with a better bass end. The difference you are noticing there is the larger cabinet. You could easily put the amps side-by-side, run them into the opposite cabinets, and hear the difference for yourself. (I think the 5:25 also has a Vintage 30, as opposed to the C90 in the 5:50. That will make a difference, too.)

So, if I play the 5:50+ and the 5:25+ in 5 watt mode, there is virtually no difference?

As ifailedshapes said, as long as the power amp stays clean an the cab is the same, both amps should be very close in 5 watt mode (6L6 has slightly more bass to my ears).
As soon as the power amp starts to clip, both amps will differ in sound because of different sonic characteristics of the tubes.

I vote for the 5:50+ head, simply because that's what I own, with a 2x12 Avatar cab loaded with a Vintage 30 and a G12H30. It is an awesome setup.