I don't think width or height matters on how long it takes to bring a certain volume to a boil. Affecting evaporation rates- yes. May I suggest going with the taller pots if one is using an immersion chiller?
And the choices we have! Decisions Decisions.
That's the fun part... :-)
After mulling it over, I think the taller pot will be important for two reasons, the most important being that I am still making small batches, basically because 3 gallons in a carboy is about what I can handle comfortably without help, and I generally brew and bottle on my own. The IC is definitely a direction I'm moving in, though with smaller batches, a bag of ice plus whatever's in the icemaker is more than enough to drop the temperature really fast.
The ten-gallon pot is really insurance for a time when I move up to other equipment equipment (pumps, etc.) or can trade beer or food or whatever for burly-man help, and can go to larger batches. Right now my 5-gallon pot isn't large enough for all-grain 3-gallon batches. I thought about an 8-gallon pot, but "in for a penny, in for a pound"--might as well get the pot that will enable what I'd like to be doing. I am also getting a step bit and a 1/2" SS ball valve setup.
The only reason I am guessing a wider pot would heat up more efficiently (and therefore faster) is that you could kick up the burner to its max without the flame going out to the side of the pot, which (I would assume) would result in a loss of energy. But I have absolutely no scientific evidence (or knowledge) to back that up.