Having a 120g with lots of fish including tangs and a trigger, I'd go the 24" width regardless
As Tony said, you can do an awesome rock structure in the middle (thank you Jon
) with swim-thrus, and the fish can swim around, over, under.
Just my own personal observations, and comparing to the fish swimming the extra two feet in my larger tank without this "width" option as much.
If you decide to go reef later, the lighting will be cheaper as well....
