I'm planning on trying out either GURPS or HERO; the beginning of this thread sold me on HERO over GURPS, but now I'm not so sure. You guys need to make up your mind!
Personally, I'm a HERO-phile. I think its much better than GURPS at being able to accommodate intermixing PCs & Critters from all of its sourcebooks.
Part of that may be because of the assumptions about the base setting. GURPS assumes a typical campaign starting at the heroic level and working up, just like D&D has for most of its 30 years (which may be a selling point for some). Unfortunately, like D&D, as power levels increase, you may get some oddball, unbalanced results.
HERO, OTOH, assumes a superheroic campaign as its norm, and scales things down for the heroic campaign. This leads to less problems when scaling up...but some powers may be gamebreaking if used in a heroic level campaign.
I would be remiss and dishonest, however, if I didn't restate that GURPS RW supplements are some of the best in the gaming industry, and are often bought by non-GURPS players as fodder for other games & systems.