Actually, people on this forum refer to these MBAs as rev B and rev C, when they are in fact both the rev 2,1 MBA. We still haven't seen an update to a true rev C. When Apple made the slight CPU bump, it seemed it was more like it was getting the CPU bump due to Intel offering a higher CPU.
I had both a B and C open a few months ago at the exact same time right next to each other. I couldn't tell any difference in anything except the printed number on the battery. The battery didn't even look any bigger to me in the newer MBA even though it has slightly more charge. It could be bigger, but it wasn't obviously bigger.
When buying a C over a B, you get a few slight modifications. One, you get an Ethernet to USB adapter when that was an option with the prior MBAs. Two, you get like 40 watt/hr battery instead of like a 37 watt/hr battery.
I believe the added battery watts were probably focused to meet Energy Star 5.0 or EPEAT Gold ratings.
I feel like the 1.86 rev B with 128 GB SSD for $1349 refurbished is the best value right now - in fact, I think it's the best value for any Mac not just in the MBA model. We learned with some other studies (true or not I am not sure) that the 2.13 is usually running at the same clock speed as the rev B 1.86 CPU even though it can clock up to 2.13 GHz. I wouldn't advise paying an extra $200 for a C high-end refurbished unless one needs it. And there is no way I would recommend buying the low-end C new with 1.86 CPU and with HDD. The SSD is what makes the MBA incredible. And we can buy the exact same 1.86 CPU as the high end B with the SSD included for $150 discount over the new rev C with HDD.