OP I feel lucky enough to own both these vehicles right now. The M3 I have had since 2010 and is an e90. I love this car and is something I am going to try and keep for as long as I can. I bought the RS in January of 2012 because I wanted the security of AWD, better fuel economy and something that I would enjoy as a DD and be able to track. Both of these cars do these very well, but I have to say the M3 does everything but the gas mileage better.
My RS is also a stage 2 and the M3 is SC. Granted the RS was way cheaper to modify and the power gains were far greater for the dollar value. As you know you get some outdated technology in the RS and the idrive is simply the best product out on the market to date (in my opinion at least).
You will be fine in the winter months depending on where you live with the RWD M. Get good snow tires and know of course what that throttle can do. I live in MA and an avid skier and I had no issues with the M in snow. Granted I feel a tad safer in the AWD in heavy snow.
Both have there benefits and specialness about them. The M just has more of that. But, the stage 2 RS is very quick and capable on the street or track, especially if you do the gen4 haladex controller to better modulate the power between the 4 wheels.
The TT has pointless space, BACKSEATS especially! So stupid and pointless unless all your friends are midgets or you only transport small children. Even then it's a bitch getting out of the back.
It's up to you. If you can keep the RS and get a CPO M do it! You will be very happy.