They are all identically constructed. The difference between them ist hat the H80 has a 120mmx120mm radiator while the H100 has a 120mmx240mm radiator and the H120 has a 120mmx360mm radiator. They take up one, two and three fan slots respectively with each step up increasing purchase cost and cooling power.
The H100 is the most popular and the one that I use in my current desktop. Previously I've used custom water solutions which while better, are a massive hassle
I don't know why people think WC is so dangerous...
I've never had a leak from either of my 3 WC'd builds. I vote custom setup is the safest. You pick the quality of your components. You can visually see every connection made to each port. I dislike these AIO WC setups. I feel they're most likely to break or leak.
Like volks1470, I also would trust my own loop over anything that comes out of a box with coolant already pre-installed. There have been enough Corsair and Antec LCS cooler leak stories around the web that dwarf leak stories from 'actual watercooling done correctly' stories.
I personally haven't found all-in-ones to be any easier to install than a custom loop, in fact I would be inclined to say the opposite since your hose length is set and mounting points limited.
Not even gonna mention custom loops just look prettier (oh, I guess I did)
H100 is pretty much as easy to install as the default in-the-box-with-your-proc cooler. And if you dont constantly tug on the tubes it *shouldnt* leak. There is no way to gaurantee that it never will... kindof like you cant gaurantee that munitions will *not* randmoly blow up in an ammo bunker. If youre careful, and do things properly, then disaster will rarely find you.