I'm fortunate enough to have MSTS and to have Auran's Trainz 2006.
Good points of MSTS: Better graphics (first thing to do is change from that stock C44-9W to the one Maple Leaf Tracks made for free), and a large popularity, thus ensuring compatability in sharing. It also has a lot of freeware stuff, and there is no real limit as to what you can do if you are able to modify your .eng or .wag files.
Good points of Trainz 2006: Easy to get into the cab and do your own thing, with multiple trains. Building your own route is a breeze, and scenery can seem more life-like than MSTS at times.
Bad points of MSTS: Crashing both your computer and your train for no reason whatsoever is common, and route editor is only for extremely experienced and patient users.
Bad points of Trainz 2006: Letting off of the throttle of some engines does not mean that you will stop accelerating (yes, naturally it also works on downgrades, but I mean on flat terrain), and opposing switches must be carefully monitored.
Whichever simulator you choose, both are excellent choices. Each person will choose one simulator or the other (I tend to go with MSTS). To help get you started for either one on freeware downloads, check http://www.train-sim.com/ and create an account. I've had MSTS for several years, and Trainz for a matter of months, and I have never had to pay for anything other than for the games themselves.
I really enjoy TRS. I have RS too. But in my opinion, (witch don't matter ) TRS is more user friendly and some of the other things like vehical physics are more real than some the other software I seen or used.