If you read maximum pc magazine, they just reviewed both cards in the november issue. The STB got the Kick Ass product award, because it has better tv out control and d3d/opengl control applets. Both score almost the same in all benchmarks, but the stb implements the tv out better, and gives control for settings for opengl and direct 3d games. If these are not important to you, just look for the one that is cheaper (last I saw, they were the same price). But if either of these are important at all, get the stb card. It also has a longer warranty, but that doesnt really mean much in the video card industry...
I say most any card that provides the RIVA TNT chipset just fine. Yes different bells and whisles to go along with warrenties. If you just want a great 2D/3D card, Creative has the AGP16meg Blaster version. Its all the speed I need right now and can get an optional TV output card at anytime since I've just freed up a PCI slot.
GO with the STB Velocity 4400. They are all TnT cards, but the difference is in the company. My experience with Diamond is that they are very sloppy when it ocmes to support and driver updates. I would definitely avoid the Creative for the same reason. The TV/out on the Velocity is beautiful, with very nice software features. I bought the creative and returned it. Running the same config with a velocity, I'm getting a much faster speed.
Plus, I've found STB to be fairly quick to respond to support issues. When I e-mail, I get a human response, not some automated mailing telling me my response will be "processed" in 2 to 3 years.