(Personally, I`d choose sylveon. Fairy types OP.)
Well, let's check the basics first, then we'll move into the stats. You've already got your grass type (snivy), but no ice type. However, if you use glaceon, you're still having a big weakness to fire.... anyways, to the complicated stuff.
Pros and Cons:
-Relatively Good Attack, Defense, and Speed base stats
-Good Attack and Defense stats can`t be utilized as well as possible without breeding and using move tutors
-Not a good wall, nor a good sweeper, any way you look at it.
-Not a very wide movepool
Altogether, you'd have to be very strategic to get a good battle-wise Leafeon. Otherwise, you're left with a Salamence with rest. It`s BAD. If you want a good, plain grass type, just keep your Snivy.
Pros and Cons:
-Good Defense, Sp Atk, and Sp Def
-Not a Wide movepool
-The movepool for Glaceon`s ice-type attacks include mainly physical attacks, making the Sp. Atk stat not as useful as It should be
Glaceon is not very powerful due to a hindering movepool and typing. If you`re looking for a plain ice type, go with glalie. Glalie is more powerful and much less hard to obtain in ORAS than a Glaceon.
To summarize, if you had to choose one, choose glaceon for type coverage if you`re for battling and strategy. However, I recommend using a different eeveelution altogether for a better team, but you can also just use snivy, replacing leafeon, and use a glalie, replacing glaceon. Both have much more promise than these two (quite weak) eeveelutions.