Good morning, I've been to both and have to say IMG is great. I would definitely recommend going there. Be sure to book online as I believe they have discount family rates/packages.
I preferred Motiongate to IMG, but that's a personal opinion and you need to consider the following:
In addition to Motiongate are the other two parks and you need to pay entrance fees for each which may make it pricey if the family decide they want to do all the parks, whereas IMG is just one place.
If your children are young, they'll probably find more to do at Legoland than at Motiongate/IMG. But if they're older, I don't think there's much to entertain anyone over the age of 8 at Legoland. If you're only comparing Motiongate to IMG, then I think it's much of a muchness.
Motiongate is mostly outdoors whereas IMG is indoors. so you need to consider the weather and how much time you want to spend outside in the heat. I'd have no problem with Dubai outdoor heat in May, but your family may be different.
IMG is closer to Dubai, but once you're in a taxi, you're in a taxi and the extra time isn't going to make a huge difference to you, I guess.Edited: 24 March 2018, 22:57