We are DVC members and have been to WDW for three long trips over the last 5 years, and to DL many times growing up and also several times over the last few years.
To answer some of your specific questions:
How many days? Somebody said two days to do everything, and if you can to that, my hat's off to ya. I guess if you get there at opening its possible. But we prefer a bit more of a relaxed pace, and three days would allow us to do most everything in the two parks.
Where to stay? Like WDW, if you want the most "Disney" experience, you'd want to stay on-site. The GC is the resort that will most remind you of a WDW resort. At first glance, the lobby looks very similar to the Wilderness Lodge. It also has its own entrance into DCA, and has some rooms overlooking Downtown Disney.
GC is also the most expensive of the on-site hotels, so if cost is a factor you might want to consider the
Disneyland Hotel or Paradise Pier Hotel. Both are more like regular hotels, though the DLH does have a nicely themed pool area, and both have some character dining. PPH is very close to DCA, but is a little furhter from DL. (Still walking distance though).
Differences?
As others have said, space is the first difference. WDW is like its own county. Lots of space and transportation options. DLR is more like an oasis in the middle of a crowded suburb. When the built DCA, they also added a lot of landscaping, and the resort feels more like a resort now than it did before. But still, compared to WDW it does feel compact, and many of the hotel rooms have views of the "outside" world.
DL is a great park, on a par with MK (some will say its better!). However, it is also older, the main street buildings and castle are smaller, and many of the walkways are more narrow.
It does, however, have some great attractions that can't be found in WDW, including Indiana Jones and The Matterhorn, and some smaller attractions like Mr. Toad and Alice in Wonderland. Pirates is stronger, and I also feel that Fantasmic is a better experience because of its more integrated setting on the Rivers of America, and also because of the Peter Pan sequence used instead of the Pocahontas sequence.
Of course some attractions found at MK are not found at DL, like The Country Bears and Magic Carpets of Aladdin.
Still, while different in some ways, DL is of the same scope and quality as MK.
DCA, on the other hand, is another story. Of course some love it, and certainly many have a good time there. However, I would rank it a distant 6th among the 6 domestic Disney parks. It lacks the cohesiveness and detail of most of the other parks, it is still short on attractions, and frankly has some real duds (by Disney standards).
Still, it does have some good rides, like Soarin' Over Ca and ToT, though those can both be found at WDW. California Screamin' is a good (though not overly thrilling) roller coaster, The Animation Building is well done, and of course there is the Electric Parade.
Grizzly River Run is also a much better raft ride than Kali River Rapids (AK). You'll also find a few other WDW clones, like Muppetvision and It's Tough to be a Bug.
Downtown Disney is MUCH smaller at DLR than at WDW, so it have fewer dining options. Its one advantage over WDW is it has a full ESPN Zone, much larger than the ESPN Club at the BW in WDW.
DLR also lacks most of the resort type activities that you'll find at WDW. Things such as boating, fishing, golf, miniature golf, etc. Of course, most of these things can be found off property, if that's what you want to do.
Also, the area surrounding DLR is of course much more urbanized than the area around WDW.
All in all, DLR is still very much a Disney experience, despite some of the shortcomings of DCA. You just have to go in knowing it will be a different kind of experience and then you should have a great time. I'd recommend any Disney fan try it at least once. After all, DL is where the parks began.
I'd say plan on at least 4, and maybe 5 days if you want to go at a fairly relaxed pace and "do it all", including exploring DTD and the hotels. Of course, you can stay longer if you like and take in some of the other activities offered in Southern California.
