I never tried the RT 2/50 (I assume that's what you are referring to). However, I think the sentiment here is that is sounds great and has a different feel than the Mod50. I wrestled with selling my Mod50 when I acquired the M4 because I didn't want to give up the extra modules and I wasn't happy carting around a second rack for the M4 and effects... In the end, I chose to keep the Mod, and add the M4 into the setup instead.
I really like the Mod50 as an amp. Alot of people have moved away to Bogner, CAE, Voodoo, M4/RT, etc. I am sure they can post their experiences with likes/dislikes. To me, the advantages of the Mod50/M4 combo are:
1. 6 module slots, midi controlled that sound and feel like they are playing through mod50. This eliminated overdrive pedals in my case (I am not a super high gain player). I use a DLX, VX, EG3/4, SL2x. Just bought COD. Hunting around for a SL and a 2nd VX to complete it. To many options at a gig!
2. The parallel loop and slave output. You don't need a minimix, or the like if you want to run delays and reverbs in parallel. You don't need a line out box if you want to run W/D.
Disadvantages:
1: you need a switcher. I am using a voodoo lab pedal switcher
2: you need to carry a second rack. I built a new head enclosure for the amp with a 5 space rack on top. It adds 8 inches in height and 2 inches in depth. But, it works perfectly, weighs less and is much less bulky than a 3rd party rack/head case.