This is a subjective opinion (regarding the EQ, not my opinion per say).
In the loop, you will be eqing the distortion. The tone has been created and you are then altering it greatly. In front of the loop, you are altering the eq of your guitar, thus altering the natural sound of your guitar. Most people agree it sounds best when in the loop. If you get really good at eqing, you can do both and alter the distortion and the guitar. (ask Chris McKinsley)
As for the C+ question, that's where most of the subjectivity comes in. The C+, or any Mesa eq in fact, is very audiophile in quality. It's excellent! Furthermore, the frequencies Mesa chooses are unique to Mesa alone, giving them a bit of a signature tone. There are others who have owned amps with and without eq and believe they can get so close with an external eq that it is not worth the cost to buy the EQ C+. (ask danyeo). So this is where you need to see how valuable the EQ is to you. Is close enough good enough? Or are you obsessed and need the real deal? (I unfortunately fall in the latter group, which hurts my wallet).
When I first got my C+, I thought an external EQ could do the trick, but then I learned all the stuff I just wrote above and have been looking to sell my Simul ever since, but have had 0 luck with the economy, not to mention that people nowadays only want EQ amps.
