Admittedly it would be an interesting twist having a guy there in power armour, and I think Zombie dude is right, a power struggle would be inevitable later on, I know who I'd stand behind if and when it comes to that.
But once again I'm focussing on the post-game a bit too much.
In terms of T-51b or the T-45d, I don't think it really matters, the T-45d model is older the than the b, and has more flaws in it, for instance in game it provides a lower agility stat, where's as the b model does not.
So if you're going to be wearing the T-51b then it was provided to him during the war in anchorage, but since the year is 2077 and it was brought out in 2076 means that they aren't a lot of them around.
So it seems more logical for him to wear the T-45d because it's a great deal more common, however not as advanced.
And in turn I think that's why it was used in Fallout 3, they actually corrected themselves because there's a decent bit of the d model kicking around, but not many b models, this means replacing and repairing parts are very hard to do.