120 is 48 x 24 x 25 Tall. Couldn't find a 100 gallon that was under 72" so 90 Long is 48 x 18 x 25 Tall. It's the more longer and wider the tank there is, the more surface area you have...so more oxygen exchange. That's not a huge factor though. If you want a tall tank though, consider how strong the lighting you will get if you're getting corals b/c the taller the tank, the more light penetration there needs to be.
All different types of tank dimensions: Glasscages.com - Glass Aquariums