The T-51b armour in New Vegas looks like it should just like it does in the original fallouts. As for female armour there would not be female power armour just power armour.
Exactly, most armours in the game are debatably 'moldable' IE, can be matched with your body figure, but T-51b isn't flexible, it's alot forged and welded components. It makes sense six is identifiable only when you see them helmetless or speak to them.
As for which I like? I like T-45d in terms of Retro feel, but the T-51b is very sleek and powerful looking, the fast differences in style I feel are because the T-51b and Chinese Stealth Armor and other cutting edge technologies, show humanity was on a possible turn from 'AMERICA!' into a modernization by our standards, and the war, thankfully, halted this, I love that 1940's feel to the game world.