Our family has been to DL a dozen times and is our "home" park. In November of 2005, we went to WDW for the first time.
As others have already said, the 2 destinations are completely different experiences so I won't even try to compare them.
You really need 2 days at EPCOT, mabye 2 days at MK, 1 day at AK and at least 1 day at MGM, maybe 2. (We loved MGM) I say maybe 2 days at MK because DL and the MK are so much alike, you may feel that 1 day is enough. I think it would depend on how long it has been since you visited DL by the time you get to WDW. (ie: Because we were at DL this past November, if I were to go to WDW next month, 1 day at MK would be enough for me.)
The entertainment at WDW is excellent and the shows really do eat up a big chunk of your time, so keep that in mind.
We rented a vacation home. 4 bedrooms, private pool, washer & dryer, full-sized kitchen with all appliances & gadgets, game room and plenty of space to spread out. It cost approx. $900 for 8 nights. The biggest mistake we made was not getting GPS in our car rental. In my opinion, if you don't know the Orlando area, it's a must.
We didn't mind the drive to the parks each morning. Like DL, they have the trams that take you from the parking lot to the main gate of each park. We went during a slow time...the week before Thanksgiving week, so the only time we encountered that "packed like a sardine" feeling was when we went to the Osbourne Lights at MGM and the last Saturday at MK when the Thanksgiving crowds arrived.
Park Hopping is harder at WDW than DL. It's doable, of course, but because the parks are so spread out, it takes much more time to do so. One day we started at EPCOT, rode the monorail to MK for a couple of hours, returned to EPCOT and hopped the boat to MGM to see the Osbourne Lights, then hopped the boat back to EPCOT to watch Illuminations. Whew...just the memory wears me out, but it sure was fun
We're planning on going to WDW again...maybe next year. Like you, we're debating whether to stay on property or not. Because we are 4 adults, if we stay on property, it's either pay for 2 rooms at a value or moderate resort, or get a cabin at Fort Wilderness. The Dining Plan is not an option for us with my guys all being very fussy eaters...nothing fancy or different please...just meat, potatoes, burgers and fries.
We also plan on going during value season again so the EMHs aren't as important as it would be if we went during a busy time. The more we debate the issue, staying on property just doesn't sound worth the extra money to us so we're leaning towards a house again.
Good luck with your planning. You're going to love WDW. Just remember...no matter how much you think you're prepared for the size of WDW...it's vastness will still take you by surprise.
On a side note: Univesal Studios and Island Of Adventure are worth a trip in Florida. We spent 2 days there and enjoyed ourselves much more than we did this last November at Hollywood's Universal. We also spent a 1/2 day at Sea World (our arrival day) and would have enjoyed it immensly if we hadn't been so tired from the 5 1/2 hour Red Eye flight from Seattle that morning.