The (Fox) Mustang GT model was a factory-installed appearence package and offered the exact same engine and suspension package as the LX, with the same performance. I don't recall the specific weight difference but I seriously doubt that the turbine wheels, 'cheese grater' tailights and urethane body parts on the GT Mustang models totaled 300 pounds. More like 100 or so - if that - and on a 3,000 pound vehicle, that minor weight difference is inconsequential.
That noted; I agree that the LX is the better looking 'Stang. I should know, I owned one for 15 years.
I suggest that you go for it.