I prefer length or width in a sump versus depth, it gives you much more surface area and usable room (square inches versus cubic inches) for equipment like skimmers, reactors and return pumps and more surface area for aeration and oxygen transfer. It is also easier to cool with a fan when you have more surface area to blow across.
One determining factor though will be your stand size if it is going under the diplay with a standard size stand. This is one reason I went with a 60" long 100G reef versus a 75 or 90G which were only 48" long, the extra foot under the stand plsu the fact I built my stand 6" deeper front to back for even more room made a huge difference on sump size and equipment placement. Rather than a 29 or 37 gallon I use a 30 long AGA tank as a sump and have lots of rom both in sump and in the stand and the operating water level is perfect for my skimmer.