We've owned 2 different hybrids, the first was an '03 Kiwi 17a which was 7' wide and had a 3500# GVWR. Our current X20E is 8 ft wide and has a 4750# GVWR. I towed both trailers with the same tow vehicle (not my current Sierra, it was an '03 Trailblazer). As far as general towing, no difference in how they towed or felt. Gas mileage was worse with the X20E but that may be due more to the extra 1000# than width. Our Sierra gets better mileage towing the X20E than our Trailblazer got.
What I can tell you is that no matter what you tow, you'll be looking at 8-13 mpg. If you were to get 12 towing the 7' wide, you'd probably get 11.5 towing the 8' wide trailer. Not a huge difference in the grand scheme of things. Believe it or not, an extra foot wide makes a HUGE difference in available floor space and would be well worth whatever MPG hit you might take.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE