I am far from a BenQ expert, but Ill do my best to answer your question and let the experts correct me on the finer details.
The difference is that you can crossgrade to the B firmware from the G firmware, but that might depend on your level of experience with PCs. It also supposedly voids your warranty. So that might depend on how paranoid you are.
From what I've read - you can go up to G7T9 [I'm using B7T9 in my drive with very good results]. The jury is still out on the improvements [or lack there of] on B7U9. Some people like it and some people do not.
Diff between 20 and 20A is one is the bulk and one is BenQ. I'm not sure which is which. As to the 1620 pro, if you put the G7T9 firmware on your drive you basically have the equivalent of the 1620 pro. It was basically only a firmware update.
Link to G7T9:
If you don't upgrade to anything else, at least upgrade to G7P9 - that seems to be the most well rounded and best accepted firmware for the 1620.
I hope this helps!