If it was a new tank and a preference of between an 75 or 90, then go with a 90. Bigger is always better.
BUT since you're considering an upgrade, I agree, don't upgrade. Not much difference, except for the height. If you have fish that prefer height, like discus, angels, etc. then maybe. Eventually, you may want to go even bigger (120 or more), so I would rather wait until you do so.