Well are you giving your dog to your mom for good, of just while you're in Disneyland. Because if it is just while you go to Disneyland sure let your mom watch your dog. If you mean for good, no keep your dog and go to Disneyland later in life.
Polar9bear - Everyone's making statements assuming you LOVE your dog. Think about it. Do you love her enough that you'd REALLY, REALLY miss her when you got back from Disney World? If not, then make the trade. If you think you would (and don't jump to any quick answers), then give up Disney for your puppy. If you really feel close, it wouldn't be worth it just to spend a day or two at Disney.