Just from skimming the docs, it looks like Leap's target keystrokes are more directly tied to the buffer's text (2 char prefix + optional discriminator), while Hop generates arbitrary labels... I can imagine the former feeling much more fluid, but haven't actually used either.

Hop user here, and have just given leap a quick test.

The difference is that leap displays all labels needed immediately after the first search key has been typed, whereas hop (more typically of jump plugins) progressively changes them as you type (depending on which hop command you're using).

The idea is that reduces the tiny delay while you identify the label on the jump target you're looking at. Say you're searching for one 'function' amongst many. With leap (default shortcuts), the moment you've typed 'sf', everything you need to jump to your target is immediately displayed. With hop, after invoking the command (with a binding to, say HopChar1), each search-narrowing keypress creates new labels which you have to then identify and type.

Seems promising at first glance, but only more usage time will tell if it's worthwhile.

I actually use both (though I use hop more often). I use hop to find a specific line, but leap (or lightspeed) to jump to a specific character.