Hey, just wanted to add my $.02. We are a GPR company in Seattle, with years of experience.
We use GSSI exclusively because we've found them to have the highest reliability in the field and the best manufacturing quality.
We use SIR 20 and SIR 3000 for imaging and a range of antennas from 200MHz to 2600MHz depending on the needs of the job.
I highly recommend GSSI, but you can't go wrong with many on this list.