Hi Tinkerbellie! We went to both WDW and DL in 2010 and we'll do the same this year and next year. In 2010, our family of five already had WDW APs and I decided to upgrade only my WDW AP to a Premier and buy DL parkhoppers for everyone else. But the Premier was only $700 at the time and it was a 3-day DL trip. We used a gp discount for our room at GCH and we didn't need the free parking ($15 value.) But there are other benefits:
1) Merchandise discount. Not only did we get a 20% discount on everything we purchased at DL, but the Premier bumped up our WDW merchandise discount from 10% to 20%. I just checked the DL website and it's still 20% for Premier (because you get all the benefits of the highest level AP at both parks.) The DL discounts expire in May, but they have extended it in the past, so hard to say.
2) Dining discounts at DL. At WDW, we have the TIW card because the AP dining discounts are so limited. At DL, no card necessary - we received a 15% discount at TS locations. And we had two TS priority seating reservations per day, so that was a significant savings.
Upgrading just one of our WDW APs to Premier helped a ton - once I did the math in 2010, it was pretty clear. This year, with the $50 price increase on the Premier, it's not as clear. I'll probably stick with WDW APs and separate DL tickets for all of us.