Care to expound?
Check out Baby Jogger's comparison chart (
link). By far the biggest difference is the size and type of the wheels. The GT's are slightly bigger and foam filled (the original is just hard plastic). When you hear Dan raving about how well the UB Vista pushes, that's primarily (though of course not entirely) due to the fact that they too have foam filled tires. The only other significant difference I know of is the GT's adjustable handlebar (the original is fixed).
We have the City Mini original for a single, and the City Elite Double (no longer in production). What we've noticed is that the Elite has a lot of great feature upgrades over the Mini, but it's huge, heavy and relatively unwieldy. It's too late now, but the GT maintains the 2 best feature upgrades of the Elite (foam tires and adjustable handlebars), but in a size/weight package a lot closer to the Mini original than the Elite. The Elite has a few nice extras that even the GT doesn't have like a handlbar console, side vent on the canopy, and magnetic flaps (instead of the velcro), but of all the Baby Jogger double options, I definitely think the GT is the happy medium - and an especially good value at the price in the OP.