If you're talking about game size, as well as load time, I think the binary format will eventually help, depending on what your setup is.
The general rule is to not optimize too early but I think this is a particularly reasonable question 'cause you'd definitely be defining workflow too.
The first question to ask is how similar the different units are.
If they're too different, using different skeletons (or "spine files) is inevitable. It's easy to make it really difficult to work with if you force really different things into one skeleton. This is NOT worth the effort of juggling setups if that's the case.
If they're really similar, you can just use one file/skeleton and use Skins to change the way they look. Change heads. Change clothes. Change equips.
On Memory:
The question of how much RAM it takes up depends on the complexity of the skeleton and the complexity of the animations. Animations are just lots and lots of arrays of ints and floats and a few strings here and there. They're not that huge, but in extreme cases (if you have lots of animations with skeletons that have a lot of bones being animated), maybe they'll add up to a few megabytes of RAM consumption.
It's much more likely that the size of the textures you use will cause your app to crash on the count of memory usage on mobile (For those not in the know, memory is shared between CPU and GPU on mobile). Note that a 1024x1024 texture is about 4MB in memory. Even that isn't much nowadays. But again, stack up those background assets and enviros and it'll add up.
On Performance:
The causes of in-game slowness are a largely different matter.
We can elaborate on this if you want.
