Well I have all three, and IMO Trainz is the cheapest, you can pick it up in Morrisons for under a fiver. It has more rolling stock than Rail Simulator available to download for it but it has the worst graphics and realism of the three. It it possibly the most detailed in terms of gameplay realism however, it can be a bit confusing for the novice user.
Train Simulator has better graphical realism than Trainz and is also realistic in it's gameplay, and also has the largest ammount of downloadable routes and rolling stock, infact a vast archive of almost everything. The game is however becoming a little dated and the graphics aren't exactly "realistic" - if the graphical realism isn't really a priority this is the best game to get, I imagine you'll enjoy it.
Rail Simulator is in it's early days, much like Train Simulator when it started out, and still needs some of the bugs and kinks working out of it and for the community to build more trains and routes for it. Train Simulator was in a similar state when it started out. It does however have the most realistic graphics going and the trains and scenes do look stunning, especially on higher spec computers with good graphics cards where you can maximise the graphical settings.
The expansions for Rail Simulator are probably down to preference, I have the Class 08 Shunter expansion at the moment and may get the Class 66 add-on next.
