Ack. What's interesting to me is that 3 weeks ago (a short while after I subbed to LOTRO), I would have voted for DDO hands-down.
I don't quite know which to vote for at the moment. I've lost a lot of interest I used to have in DDO in the past month (i.e. I've logged in maybe twice) and instead have started my first month of LOTRO. Relatively, DDO is more fun for me when doing something new or challenging, but that's incredibly rare. At the moment, the sheer amount of things to do in LOTRO combined with the "newness" factor has made the game very fun for me.
In terms of overall design, I would say that LOTRO probably wins due to sheer size and its amount of quest chains, which I enjoy. In addition, its "fluff" such as searchable auction houses, personal homes, crafting design, armor's influence on appearance, and area design have made the game seem even more expansive. I have yet to encounter the feeling that there's nothing to do in that game, unlike DDO.
However, I find the combat system in DDO to be more interesting, though arguably less complex than that of LOTRO. I personally have never played Asheron's Call. Sadly, that combat system and the handcrafted instance dungeons are the only real things DDO has going for it right now.
For me, at present, I may have to vote for LOTRO. However, when Mod 9 rolls around I'm going to definitely be playing DDO most often, though I doubt I'll end up cancelling my LOTRO subscription (Just as I haven't cancelled my DDO sub when playing LOTRO).