You couldn't pick two more different places to compare on the planet. The only real things they have in common are they have an airport and there are clubs there. I've been several times, and don't mind it really, but it's very different from Ibiza.
Here's a brief rundown of the Vegas experience:
- It's in the middle of the desert. In Summer it's 40-45 degrees C in the heat of the day.
- Clubbing isn't the #1 draw. Gambling destination first, entertainment (including clubbing) second. I will say there's a ton more to do there than Ibiza, and the food overall is very, very good if you go out looking for quality. And that doesn't necessarily mean expensive, you can eat well on any budget.
- Clubs host top EDM and Hip Hop talent (which is fine, I do like some EDM DJs). Every once in a while Daylight/Light (Mandalay Bay Casino) will bring in someone a little more off the beaten path, i.e. Coxy and Above & Beyond. Otherwise there is absolutely no underground/techno scene to speak of.
- Vegas is most known in the states for the pool parties - that's kind of the unique experience there. Think Ushuaia but a bigger pool that you're actually allowed to swim in. Pool parties run from 11am-7pm.
- Vegas clubs shut down between 430-5, for the most part. Most of the time you have to find an "afterhours" (strip club, smaller lounge) to have a true dusk-dawn experience.
- All about the glitz/glamour/VIP. Clubs like Marquee and Omnia leave very little room for people who aren't bottle service.
I liken Ibiza more to Playa Del Carmen/Cancun, Mexico or even Miami than Vegas, really.