As I understand it, the REV B GPS was introduced on later production Solos. It can be identified by removing the GPS cover and checking the color of the PC board. The REV A is green, the REV B is black.
From what I've read, the REV B had improved shielding, and a problem with the REV A not properly retaining the almanac data from previous flights was addressed.
Otherwise, they are both basically the same. Both only receive the United States GPS constellation. The REV B has appeared on the 3DR store occasionally, but it never widely available as an upgrade. You can often find them on eBay, Amazon, and even the classified section of this board. Whether or not it is a worthwhile upgrade is a matter of some debate. Some have had very good success with REV A boards, but most have issues with long waits to get a GPS lock. In general, it seems that REV B board get a GPS lock a bit faster.
Many Solo owners here insist that a high-quality M8N GPS (such as the one from mRobotics) is needed. Others will say that the original (particularly the REV B) is perfectly adequate.
I have three Solos, and all came with REV B boards so I've never been able to compare directly. I recently installed a cheap ($18) M8N from Banggood in my backup Solo and it definitely gets a lock much faster and gets a lot more satellites. It does seem to be somewhat sensitive to multipath interference in cramped areas though. I can lose GPS lock and still have 15 satellites. The moral of the story here is that more satellites isn't always a cure-all. Noise immunity and multi-path rejection are probably more important.