In 30 years of gaming I've owned about 100 RPGs, and played a few more- including some that never saw publication.
HERO is my favorite system of all time.
My personal experience with GURPS is limited to 2nd/3rd Eds, when I played with a lot of guys who were, among other things, GURPS playtesters. I understand from past discussions with Jürgen Hubert that some of my complaints have been fixed in the current edition.
HERO
Pros: Most flexible RPG I've ever played, bar none. Near-total genre transparency- a PC can be plucked from a traditional FRPG setting and placed into a sci-fi setting without any real rules changes. Once you jump the hurdle of PC generation, the game tends to run quite smoothly.
Cons: Some of the math in PC generation- while only simple addition, subtraction, multiplication & division- can be intricate at times, but there is a chart that makes it easier...and many Excel spreadsheets exist to make it simple to keep track of points. The game's very flexibility can cause confusion in PC gen simply because there may be thousands of ways to design a PC based on a particular concept.
GURPS
Pros: The best set of supplements in RPG history in terms of research, well-thought out design, and comprehensiveness. There is probably a GURPS supplement for any genre of role-playing there is. They're so good that, even though I dislike the system, I actually own and keep track of GURPS supplements.
Cons: Sometimes, the core rules differ from those in the supplements, or from supplement to supplement, reducing the system's claim to being "generic" and "universal." Occasionally, some of the powers are really expensive for the payoff- I remember 2Ed/3Ed Telekinesis as being extremely overpriced for the power you got.
and if I may...
You may have also seen a game called Mutants & Masterminds- if not, I suggest you check it out and give it serious consideration. Its nearly as flexible and internally consistant as HERO, but its mechanics are generally less mathematically convoluted. Its also evolved from the D20 system, so you and your prospective players will probably find it easier to learn than either HERO or GURPS. Its rules-light(er) approach to the D20 system reminds me a little bit of the Marvel Super-Heroes RPG, which for some may be a plus or a minus.
Like any really good superhero game, its capable of modeling PCs in any RPG genre, since superheroic settings are such a catchall of character concepts.
With each of the 3 RPGs listed above, all you really need to play is the single core book, your dice, and pencil & paper. However, each of the games also has an extensive product list to make play easier- gadget & equipment books, collections of foes, settings, etc.