Leafeon is a physical attacker with high Defense and decent Speed, and Glaceon is a special attacker with decent defensive stats. However, Glaceon is slow, and it doesn't have many strong special attacks. All it gets is Ice moves, Shadow Ball, Signal Beam, and some weak moves like Hidden Power and Water Pulse. It also has only one resistance as opposed to 4 weaknesses, so it's not that good at taking hits. And lastly, it's available really late in most main series games, if not all of them. By the time it's available, I usually have a strong Water type on my team, and they can also learn Ice type attacks. So Glaceon isn't really worth it for me.
Meanwhile, Leafeon has 4 resistances as opposed to 5 weaknesses, and it has actual Speed like I said before. And while I tend to learn towards special attackers when I choose my Water type, I can go for either physical or special when choosing a Grass type. And in Pokemon Platinum, as soon as you receive your Eevee, you can head back to Eterna Forest to evolve it into Leafeon (even if it takes a while to get there).
So basically, Leafeon wins for me.