Here's a link that gives some comparisons:
http://www.seiner.com/WestyFAQ/index.php/AboutTheWesty
The site is a van conversion site: their opinions will be based on the vehicles will be a bit heavier than standard (after conversion) I would think.
With the E150 tow capacity at around 6400lbs it is more than up to the job of towing a 2720. My list would go like this:
Everyday use, driveability (more car-like ride, etc.) E150
Gas consumption/weight E150
Most towing of a TM2720 E150 or E350
Towing at high altitudes/steep grades (e.g. Rockies) E350
Very heavily loading the vehicle up with people and gear E350
Some maintenance items (e.g. brakes) E350
Getting more vehicle for same money E350
For my use, I would want to tend towards the E150 because most of the vehicle's use would be everyday - I would want more driveability and better mpg - and I wouldn't be towing over the Rockies with a full vehicle much. However the decision would still be tough. I would want to drive both first.
Your criteria are different, I'm sure.
This probably isn't much help
-Paul