I try not to compare the two and think of them as two separate entities. That way I'm happy at either one.
Personally, if you think you may eventually visit DL, I would do DL first, then plan for WDW later. It seems that people who go to WDW first, then go to DL are disappointed, expecting the same kind of atmosphere.
If you were to give me a choice and pay for it for me, I would definitely choose WDW! WDW is more costly because it requires more time, and for me it requires staying on property (which costs more). Plus, I'm in Hawaii which makes WDW twice as far away as DL. I go to WDW every few years, and on the years I can't make it there, I go to DL.
Also, as others have mentioned, the rides, although named the same (e.g. Space Mountain, etc.) are different. Space Mountain at DL is better because of the sound effects. The shows are also different. MGM's Fantasmic! is neat because it has seating, in DL there is none. However, the show itself is different, and a lot of the people I know (who went to DL first) think DL's Fantasmic! is much better than MGM's. Personally, I like both and look at the good aspects of each one as I'm enjoying them.
Anyway, there are a lot of factors to consider, and I probably haven't helped any with your decision! I just think that if you eventually plan to visit both, DL should be first.