Like bb said, the two resources are very different. I've been converted to the Jeff Sackmann philosophy of studying fewer questions in greater detail, so I'm not a big fan of GC Tests or
MGMAT. However, there are people that really like both of them, so they certainly aren't bad resources. If you really had to choose between the two, I'd recommend
MGMAT because you're going to need some CATs to supplement GMATPrep. If you're taking the test in less than 2 weeks, the GC tests might be a better choice.
I prefer Jeff Sackmann's Extreme Challenge question set over both of those resources, but it only contains 100 questions. However, the questions are all very GMAT-like and cover every 700+ concept that you'll need to understand for that Q50 score.
In my opinion, you shouldn't be using GC or the Extreme Challenge set if you aren't already getting 46+ in quant.
Good luck and welcome to GMAT Club!