I can attest to the E90's ability to haul a family around. I did it all the time. I don't think you'll be too horribly bad off trying to park an X5M, my landlord has an X5 and doesn't have any problems, but the mileage will make you cry. I thought my M3 was bad. An X5M is much, much worse.
I agree with selling the E92 simply because getting a car seat in and out of a coupe sucks. It's very doable, I did it when my first was born, but I wouldn't recommend it if you can just as easily do the sedan.
My $.02
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