IMO, base the decision on the type of fish you want to keep. If you plan for a tang, then go with the 125. The lateral swimming room will be a plus. The depth (front to back) of the 120 would give you a lot of leeway in terms of creative aquascaping though, so if you can select fish that do well in a more compact system, this might be the way to go. Longer tanks are nice, but unless you really get into some monster sizes, the depth is restrictive to a degree.