The developers will be supporting both companies. Given how small a niche market WWII Flight sims are these days it would be suicide to not support both companies equally, on all cards that are able to run the game.

Some people were saying that ATI will not be as well supported at Nvidia, but the announcement that XFire will be up and running sooner than SLI shows that they have no preference. Luthier even said that the office was split down the middle with ATI and Nvidia fans.

Get the best card you can afford. Look at some reviews and some benchmarks, ask anyone on these boards with the same or similar card how it is working for them.