Don't know if this has been brought up, but it also depends on where you'll be brewing.
If brewing outside, getting the larger pot for better efficiency is the best choice. It's cost effective, future-proof, etc. If you're brewing on a stove indoors, it may not be possible to scale up in such a way, limiting the pot and boil size.
It's probably safe to assume many people here are not NYC apartment dwellers such as myself, thus making a 10-15 gallon pot an easy buy. This may not be your case, but it's just some food for thought.