We used Costco and were happy with it.
You can buy the meal plan, such as it is, through Costo (or
AAA or getawaytoday, I believe, since they all go straight through WDTC...when you call the Costco Disney people after you've booked, you're actually talking to CMs in Anaheim that just answer their phone with the Costco brand added on).
The main components of the package are basically the same...if Disney is offering a "dream coin" still they'll likely all have that. Each company will have a lanyard for each person that you pick up at certain stores with a voucher, along with a commemorative pin. With Costco you get a "free" (not with my package...it priced out to about a dollar less than AAA + a character b'fast for each of us) character breakfast, Preferred Seating vouchers for some shows in DCA, and vouchers to get you in to Mickey's ToonTown Morning Madness. The others have slightly different things...like with getawaytoday you get 2 Magic Mornings assuming your package is at least 3 days long in the parks, but you don't get MTTMM vouchers.
You just have to know what you want, grab a notebook, and check out prices and exact things in the packages, and decide what's important to you!
And FYI, if you're booking an onsite hotel, the extra MM day with getawaytoday isn't important, b/c you'll get MM each morning it's offered during your stay just by staying at the hotel.