Espeon for me as well. I've played through Colosseum several times, I hardly ever used Umbreon once I got another pokemon, like Flygon. Don't get me wrong, I love Umbreon, he's usually always on my team at some point in the handheld games, as long as he's been EV trained and has a good nature and moveset. But with Colosseum, Espeon all the way. Confusion usually one-shotted everything, if it didn't, it would be taken out the next turn.
In terms of looks, I can't decide. .-. I've always loved Umbreon, and shiny version is AMAZING.
Whereas the shiny version of Espeon... Is odd, to say the least. >.<
I may be a bit of a hippie, but there is a line that cannot be crossed when it comes to colors, and that line wasn't only crossed, but beaten with a stick. D: At least, it was to me.
All in all, Espeon > Umbreon, at least in Colosseum.