This is Fallout, not Dress Me Up Barbie. Personally I see nothing wrong with the appearance of the T-51b. Frankly I think it looks better than any of Bethesda's Fallout 3 power armor designs.
I have to agree on that, I like the look of T-51b, better than I do the Brotherhood power armor in Fallout 3, in fact, in Fallout 3 it was my favorite power armor.
But we don't necessarily need to have only one, both armor types could be included in the game, maybe T-51b is the stronger of the two reserved for higher ranking Brotherhood members, and T-45d is the weaker one, but if I we can only have one, it should be the T-51b, not only do I like it's design better, but it also seems to make more sense, being as this is the same Brotherhood we got in earlier Fallout games, I'm not even sure why Bethesda felt the need to change it in Fallout 3, even if the East Coast Brotherhood wasn't as well equipped, I see no reason why they couldn't at least have a few suits of T-51b left over.