A.L.
I don't own one yet, but I also have it on my shortlist of strollers I am considering. From what I have learned I would spend the extra $60 for the GT model.
When I tried it out in the store the regular City Mini felt rickety on uneven terrain the wheels look and feel cheap. Now the GT does not have pneumatic wheels like I would prefer, but they are definitely better than the regular wheels. If you are only using your stroller on absolutely perfectly paved surfaces and inside (like the mall) - I am sure the regular wheels will be fine. But and gravel, grass or playground mulch, I would want the GT.
BTW - even though the brand name is "Baby Jogger" none of their "City" models are meant to be jogging or running strollers...