We have AP to DLR and go there a lot. We've also gone to WDW many times over the past 10 years or so. (see siggie).
For us
Disneyland is our first love. It's the original. It's one of the reasons why when we go to WDW we don't spend that much time at MK. DL is just so much better.
Since we basically have DLR in our backyard we get asked all the time why we go to WDW? When we go to DLR we just go for the day (or partial day). We don't stay onsite. We go home at night. If it's crowded (and it really can get very crowded) that's okay-we go home knowing that we can can back again soon.
DLR and WDW have some similarities and some differences. There are many threads that address this so I won't here. Well, just one.
Disneyland and DCA are really close together! You can walk from one to the other in a minute. Seriously! It means you can go back and forth during the day without taking a lot of time like PH does in WDW. We were just there yesterday and we went from DCA to DL to DCA to DL and back to DCA to end our night.
If you can swing it financially, I think Disneyland is not to be missed. If I had to pick the best time to go I would have to take some things into consideration.
#1-do you want to be there during lower crowds?
#2-if you go during lower crowds, are you okay with shorter hours and less entertaiment options?
#3-do you want to experience the summer experience?
#4-how about Christmastime at DLR?
#5-Halloweentime?
If I had to pick one time to go I would choose Christmas time. Early in the season, early December. DLR does a great job with Christmas. And unlike WDW you don't have to go to a hard ticketed event to experience special holiday touches (like the snow)
If you want to go during lower crowds Jan and February are good times. (not during President's weekends)
If you have any specific questions, just ask.