Brand loyalty is highly over-rated. I go with whatever works for what I plan to do. In my case, Intel was better so I went Intel for my personal box. My budget-box, which I use for testing, doesn't need much. And heck, it's a budget-box, so AMD is my choice for that.
As for ATI vs. Nvidia...I've had marginal success with ATI drivers. I think I've only had problems with Nvidia drivers once so that left an impression with me.
It shouldn't be slow....specs please?