I have both. I bought my girlfriend the bagboy GT 2 years ago, and I ended up using it a bunch of times myself. Last season, I needed a new bag, so I bought myself a sunmountain speedcart and the speedcart bag that is designed to work with the cart.
I would recommend the Sun mountain, because it folds up nicer. It ends up being slightly larger, but you don't have to take the front wheel off like you do with the Bagboy. It really becomes a pain in the ass.
Also, the Sun Mountain has a hand brake that operates the front wheel. It is much easier to engage, but you need to make sure that the cart is directed downhill otherwise it will role backwards. The bagboy has foot breaks which can be a little tricky to engage on hills, but you don't have to worry about which way it is facing on the hill. The Bagboy cup holder is also better.
Durability wise, I haven't had any problems with either. They really don't need to take too much abuse.
That is about it. You should be happy with either one.
