Well, they're two completely different things. Depending on the age of your kids and what you are going to want to eat may help you decide.
MBYBBQ is an outdoor seating under a pavilion with an all you can eat picnic style food like dogs, burgers, chicken, ribs, mac & cheese, caesar salad, baked beans, etc. Plus unlimited Mickey ice cream bars. Alcoholic beverages are also included. Kids can dance on stage with Mickey, Minnie, Chip, Dale, and more. It's a ton of fun for kids. But as they start to get older, you'll find that the BYBBQ won't appeal to them as much.
If your kids are still young, I'd do the BBQ before they get too old and lose interest.