Leafia is obviously the much better choice of the two eeveelutions...
Glacia got the strong defense and a praise-worthy sp.atk, but sadly with such a low hp and speed, it really has to take too much beating in order to get to its turn. Without an agility or mine cart passed to it, it will be completely destroyed with all those arm hammer and stone edge out there... The low hp just kills it too badly. It got the same problem with Flareon.
Leafia got a decent attack and speed, with a powerful defense to take even a special effective physical attack at least once. Access to sword dance makes it dangerous and capable of baton passing as well. The only thing making it inferior to the older eeveelutions is the lack of physical movepool that is given to all the eeveelutions. Leaf Blade and Scissor Cross (along with Return which everyone knows) are the only physical attack it got. In comparison to many other Pokemon in DP who got access to at least 5 different high power elemental attacks, it is rather overly predictable how it is completely useless against any flying types... Let's not forget how its ability is crap-tastic.
The order of power is still Jolteon/Vaporeon at the top, then Umbreon following with its tanking ability and its only weakness being fighting attacks. Leafia follows, with Flareon/Glacia in the end.