I think it just depends on how well you love each resort and how you intend to propose. DLR and WDW are very different. I prefer DLR and find it romantic, but I can see WDW being romantic, too. I wouldn't choose DLR, though, if you haven't been there yet. Everyone is different. Some DLR and WDW vets prefer the first parks they visited, some prefer the parks across the country from them.
Since you haven't visited DL yet, I'd suggest popping the question at WDW, since you already know that place and thus will be able more easily to plan the big ask (location/timing/manner/etc.) I say that because sometimes, even though you may want to visit the other parks, you turn out not to like them as much as you anticipated. (That happened to us with WDW.) And that could put a damper on things.
On the other hand...if you're saying (if I'm reading correctly between the lines) that you don't find MK to be particularly romantic, then you might want to go ahead and make the ask at DL.