I thought that the 2008 Hulk movie was a reboot/remake of the less-than-perfect 2003 Hulk movie, but it is more like a half-sequel/half-reboot with different actors. It is also partially a sequel to 1970's Bill Bixby/Lou Ferrigno TV show, which I really enjoyed back then.
My advice is to watch the 2008 version and skip the 2003 version. I find it interesting that in the end, "Tony Stark" makes an appearance, which is a shameless plug for the somewhat enjoyable 2008 Iron Man movie. I could not find the "alternate beginning" on the DVD, but supposedly it is on the disk somewhere and it has a brief tie-in to the 2011 Captain America movie. Somebody was thinking ahead.
I always wondered why the Hulk never ripped his pants. The 2008 version tries to explain this a little, but it is something that we mostly have to take on faith. Another problem I have with the physics is the Hulk appears to weigh a ton or two, which is a complete violation of the laws of physics when you start with an ordinary man. By the way, gamma radiation doesn't turn you into a green monster; It kills you.
Unlike the 2003 version, the main actor is not completely dull and the movie proceeds at a fast enough pace to keep us interested. Although it is a comic book story, what matters to me is that I enjoyed the characters.