I would suggest the nVidia.
From experience, I had an ATI Radeon 9200 and later a Stealth X1550 from Diamond. Both ran almost everthing fine.
Except for Trainz.
The 9200 worked fine in WinXP with TRS2004 but not great.
The X1550 with Vista for some reason caused problems with the model viewers, TrainzObjectExplorer and FileViewer.
I also noticed all the Trainz versions I have in the settings there is a box to check for some kind of ATI fix.
Since getting my GeForce 8500, even though not the most modern board, it runs everything like the ATI boards and Trainz programs run better plus there are no model viewer problems.
If it wasn't for whatever weird reason Trainz appears to prefer nVidia, I'd suggest ATI, but to run TRS try nVidia.
Make sure to check power requirements and DirectX version support. You don't mention whether you run Vista or XP, if Vista it would be better to find a card that supports DX10. Trainz runs on a lower version but having that could help other applications.
It also helps to check with the computer manufacturer about what boards they suggest, some may not be compatible for some reason and the manufacturer would have already checked into it. I was directed to the 8500 after talking to the manufacturer of my desktop about getting a replacement, just be ready if they try to ask you to buy one from them, I preferred getting one from the store physically, that is just how I wanted it card in hand.