I just came from the Galaxy S II before switching to the iPhone 5.
Overall, I'm much more satisfied with my iPhone 5 than Galaxy S II. I've used the S III before and it's really close against the iPhone 5, but the S III is quite a bit better than the S II. The worst thing about the Galaxy S II was the battery life. It would be rare that I'd be able to make it through a full day, especially on LTE. The battery would die quick, even after I purchased an extended battery pack. The iPhone 5 has a much better battery and lasts longer.
When I switched to an iPhone 5 the first thing I noticed was the processor. It's a lot better than the S II. Websites load faster, it's quicker to navigate, takes pictures faster, a lot smoother and runs games/apps with no lag at all. An example of this is how the game Temple Run wouldn't run well on my Galaxy S II. It didn't run bad, but not well. It ran at about the same lag as my iPod Touch fourth generation. There was a bit of lag that could mess you up and it wasn't as smooth as expected. When switching screens, you might think it does it fast and smooth - but it doesn't. I thought it was fairly fast and smooth but my friend, who had an iPhone 4S, pointed out how slow it was and when I used his iPhone it was a lot faster. They're fixing this with Project Butter in Jelly Bean but that won't be available for the S II for a while.
Another thing was listening to music on the Android operating system drains the battery fast. A lot faster than it should.
All in all, if you have a decent upgrade price, go for it. You won't regret it. It's a lot smoother, feels a lot better in the hands, better processor. You'll love the iPhone 5.