I guess it comes down to what you want to spend and more of what you looking for in tone.
I currently own an RKI and love it to death, I havent played an RKII but i have played the roadster which is suppose to be a striped down RKII.
If RKII's do sound like the roadsters, then it sounds more like the old 2 channel recto's (darker) but not as clear and tight. What i mean by darker is not as much mid's.
Now as the RKI, i have owned 4 and still currently have one that I will not sell. This amp is a mix between a dual recto and a tremoverb, not as much upper mids as the tremoverbs, but still more then the duals.
Other changes between RKI and RKII, the RKII has a better clean, not much better, but it is better, either amp has a great clean though compared to all other rectifiers.
On the RKII the Brit channel was revoiced with added gain. Honestly I have played the roadster, same brit channel and my RKII and I much prefer the Brit channel on my RKI, the RKI ismore towards the british voiced amps we grew up and loves, and with a booster pedal you can get as much gain as the RKII's.
Other differences, RKI has one parallel and one series loop, RKII has two series loops. RKII has a tuner out where the RKI has switched out.
As to cabs, honestly I much prefer a recto traditional cab, i do not really care for the C90's or having a partial open back. I have had the RK cab and unless you are a pro more then likly any place you play will not give you two mic's anyway to mic each side of the cab. So to me it was useless and my traditional cab just sounded so much better!
So it all comes down to taste and what you want