Everything matters and is the degree that you seem to be asking about
Aero (the Cx number) of the frontal and the sail area of the sides
The tires of both the TV and TT. What cype/class, rating vs actual weights, tread,
how aired up, rim width vs tire recommended range, etc
Pure weight affects mostly the getting up to speed and back down in speed and of
course inclines/altitude
Finally...the drivers right foot...AKA how you drive or your driving style
Performance is more important to me than MPG, though MPG is important enough to
be on my have to have list
Aero in that one trailer might have a better frontal profile than the other,
taller, wider, etc. Also on Aerodynamics...the undercarriage makes a difference
on MPG. Turbulence if there is lots of stuff down there to create vortexes
Personally, think the MPG differential will not be that big a deal for 'me'...