It depends on what you're looking for in a collection; gaming ability or fluff/model reasons.
Certainly the DE are the more powerful codex; the eldar rules are alright, but they are mostly hampered by points cost. For example, Dark Reapers have Heavy 2, Str 5, AP 3 Reaper Launchers, but when they're more expensive than much more adaptable Space Marine Devestators with Missile Launchers, then you tend to suffer with overall numbers a little. The ability to fit an extra squad or two into an army would greatly enhance every Eldar Army.
If you wanted eldar, they're one of those Codexes that should be review over the next few years; certainly I am looking forward to having an army that will be ready to field with some shiny new rules.
Although if you still can't decide you could go halfway and get a squad of Harlequins to start with