I used to sell and install Wilson antennas and amplifiers. I had very good luck with them. One thing that I appreciated was that their manufacturing was done in-house in St George Utah. And..... They fed me lunch when I stopped by..
One thing that I did find is that, with CDMA carriers, when you get so far away from the tower, even if you manage 3-4 bars, calls will fail. I puzzled over that for a while. I sat at a remote homesite with all of the Wilson gear installed. We had a yagi antenna pointed at a distant cell site that hosted both Verizon and AT&T. Customer had 3 bars on his Verizon phone, I had 3 bars on my AT&T phone. My phone worked great. Customer, with 3 bars of signal, could not get a call to go through. Over time I saw this happen on a number of installs. I finally got the scoop from an engineer who works for a local cell carrier. Apparently part of the functions of the CDMA system watches latency between the tower and the mobile device. If the latency exceeds a certain threshold, regardless of signal strength, it will drop the call.
Short version : amplifiers and gain antennas will help whether you have GSM or CDMA service. But, at longer distances the GSM will benefit more than the CDMA.