At WDW, I would insist upon staying onsight, and would generally want to pay the big bucks to stay at one of the delux resort for at least part of my stay because:
1. The resorts are awesome!
2. The convenience and time savings is significant, and I never have enough time at WDW to begin with.
3. Added benefits including the extra hours are significant.
4. I can't imagine leaving the magic of the World once I'm there
At DL, I usually stay onsight part or all of my trip to DL because
1. The resorts are more fun
2. The resorts are nicer than the places directly across from
Disneyland, although, as Daannzzz points out, many of these places are fine.
3. The onsight resorts are close enough to avoid ART or hotel shuttles while the nicer hotels in the area require transportation to and from the parks (although most of these shuttles aren't bad).
4. I can get the multi-FastPass benefit when onsight packages are booked through
AAA.
5. I can sometimes find a pretty decent rate onsight by booking at the right time; I might make a reservation elsewhere that can be cancelled without penalty, watch the rates on Expedia, and then book onsight when and if discounted rates are released, usually a couple of weeks out. Then I have to remember to cancel the first reservation.
6. I am very spoiled.
In summary, the added cost generally does not support the benefit of staying onsight at DL nearly as much as it does at WDW. If you can get a good AP or other discounted rate, then it's worth staying onsight for the magic of it all.