+1 to everything that's been said. I think the most important factor is what do you plan to use the tank for and how willing are you to love your tank (AKA how much maintenance and care are you up for). If you are already knowledgeable about saltwater tanks, please forgive the following, but in case you're not...
Larger volumes are definitely easier to keep your water quality stable in. In my experience, I love this about my 90 g (as opposed to the 24, 30, and 60 that I've had) and I want to go up to a 150+ as soon as it is feasible. It's also true that as a coral addict, I need more real estate.
If you have fish only or fish with a few soft corals, you wouldn't need metal halide lamps. There are a number of different fluorescent lights that could light your tank well. In my opinion, live rock is a must but lighting is not an issue on that score.
For me, aesthetics are very important. That said, sometimes the space you have just looks better with one tank shape vs another. Oh yeah - think about access to the plumbing and working parts. As we are reaching around under the tank to fool with skimmer etc, we wish we had thought a bit more about that when we positioned the tank.
By your screen name, I'm curious if this is a home tank or a showroom tank. There are some local fish stores that offer tank maintenance service. The one I know of is Southwest Aquatics.
Finally, there are a lot of folks on this forum who know waaaaay more than I do, so this is a good place to field questions.
Good luck!
. If you're stuck on 125 or corner or something, definitely 125.
