I think each has it's plusses and minuses.
At DLR, I love how close DTD is. It's leterrally a few steps away from the main gate to both Disneyland, and Disney's California Adventure. If you park in the parking structure, you could skip the tram after you are done at the parks, and stroll through all of DTD.
At WDW it is not near any park, so it usually has to be added on for another day, and many people skip it for that reason. The best day to do it is on your Animal Kingdom day, since they close now at 5:00, there is still plenty of evening left.
DLR's DTD is more along the lines of the feel of Westend. To me Westend is the non Disneyside, as Marketplace has mostly Disney shops. The only real Disney shop at DLR's DTD is World of Disney. Everthing else is very un-Disney. With AMC, Staribilla's House of Blues, and Rainforest Cafe being the only other common entities. Although there are rumors Staribillas is leaving DLR.
WDW's Downtown Disney clearly has more to do, besides offering Pleasure Island and Disneyquest, they offer small tidbits like a Mcdonald's, a Lego play area, a train ride and merry go round for the kids, as well as Cirque De Soleil.
DLR's Downtown Disney is more geard to adult's, with a movie being about the only option for the under 21 age group. They are also bigger on the resaurant offerings there, having Rainforest Cafe, ESPN Zone, Ralph Brannigan's Jazz Kitchen, Y Arriba Y Arriba, House of Blues, Napes, & Catel.
Most people feel the DTD in DLR is the best part of the expansion , and I agree. It really did make the place into a resort, more so then the offering of two parks did. Many hang out at DTD that don't even visit either park.