What are the exact dimensions of each?
The front-back glass depth, IMO is a worthy consideration. If going for a full-scale reef, IMO the more front-to-back space you have, the more aesthetically pleasing to my eye.
When making a live rock landscape, having a 12" front-back tank makes it difficult, even with 18" I wish I had more. While for fish you may want a long tank for swimming - for aquascaping that's tricky.
Also, for lighting a reef - a longer tank may require more bulbs/expense ... and then power bill. I would pay attention to finding a good light fixture before getting the tank - 60" IMO is a better option but lighting might turn to be more expensive.
Depending on your lighting plans + stocking plans - maybe one makes more sense.
Personally, I lean toward the 4' x 2' 120, as the aquascaping gives more options - and for lighting 2 MH will do the trick, where more would be needed if the tank was longer. But that's so I can re-use my current lighting. If I used just T-5 or VHO - it might be less of a difference - your lighting/stocking plans might not be impacted as much.
That's my take on tank-dimensions. Many others out there, I'd encourage you to find as many as you can

You just might have a different idea of what you want than I do of what I want.