75 gallon vs 90 gallon

The only reason I didn't go with a 90g over the 75 is because of the height, already reaching down to the bottom of the tank is a chore being 6' tall, especially with the lighting over the tank. If you don't mind dealing with that and have adequate lighting to hit the bottom of the tank then go for it.
I agree that the 90 doesn't offer you a larger selection of tangs although it might be better for some types of fish but not too significantly IMO. Most tangs need straight line swimming room, not vertical room. It's a great tank though, enjoy.