You know at first I preferred Espeon back when I thought Umbreon was just...weird and useless. Then in SoulSilver I decided to get an Umbreon because for some reason I started taking a liking to them. I trained my Eevee for about 6 hours straight till 3 a.m. (I was on again off again playing) till it evolved at lv.30 to Umbreon..Then from there I was stuck with figuring out how to use Umbreon correctly.
Due to me not knowing much about Umbreon I didn't pay much attention to its stats, meaning I didn't give it the right moves that complimented its high defense. I used physical attacks and it got me nowhere, and I started hating the very Umbreon that I spent hours trying to get. Umbreon went in the PC and when I started playing SS again after like 3 weeks of hiatus I took umbreon out. I know knew more about Umbreon and how it was more of a 'status' pokemon instead of one meant to attack. I gave it moves at first like Toxic, Taunt, and confuse ray. Later deleted taunt. From then on I realized how awesome my Umbreon, and others, were and how stupid I was to not pay attention to his stats or even try to properly raise him.
Now due to neglect my Umbreon is at lv.54 (still not bad) and I'm training him to get his levels up. He will definitely be a permanent member of my SS team, and I just wish that when I first started training that Lv.5 eevee I got that I knew what I was actually doing. So to sum it up, to me Espeon is a cool pokemon and really one of Eevee's best evos, but in the end if someone wants a pokemon with a high def/sp.def and will definitely make battles with you a pain for your foe..then choose Umbreon :D.
Though when I buy HG I'll probably get Espeon since I already have Umbreon.