As far as I'm aware (and stand to be corrected on this), most of the differences between A, B and C units were firmware based. I know that a B version unit can have the latest firmware designed for the "C" model installed (which effectively converts it to a "C" anyway).
I have a "B" model, and am running with firmware 1100. Darren (mrotygttdi, the RNS god), advises that if you have a CAN Gateway older than revision "L" (which I think is build later 2007 early 2008), then running a later firmware will result in battery drain.
As I said, I have the "B" version with 1100 and my CAN Gateway is on old "E" version. I've left my car for 5 days and no battery drain issues at all. Apparently the only thing that my "older" firmware doesn't give me over the later 2600-2800 series is support for 32gb SDHC cards (standard is only 2gb CD), no speed alerts and no voice control (using the optional voice control upgrade accessory). These are not biggies for me, so am happy to run with my 1100 and save the hassle of upgrading my CAN Gateway.
I also believe that the newer Version "C" units are shipping with LED screens which apparently give a crisper screen display (although I can't really moan about my old povvy screen in my "B" unit.)
Hope this helps.