Both are excellent microphones. I own Gefells and have heard more AKG tapes than anything else (probably because they are so popular). It really comes down to price and what you can afford. If price is an issue, get the AKGs. You will not be disappointed. If money is no object, get the Gefells. I prefer the Gefells to AKGs. Whether you can stomach the price difference is entirely up to you. This was an easier decision to make before Gefells started to significantly raise their prices over the past decade. The good news is that each microphone holds its resale value well so you can get rid of them easily if you decide that you do not like them.
There are a few other factors that go into the calculus. Customer service is one of them. I dropped a Gefell capsule once and had to have it serviced. I could not send it anywhere in the U.S. so I had to send it back to Germany for a rebuild at a cost of 400 Euros. Expensive mistake to be sure, but you may want to investigate whether you can have AKGs serviced for lower cost or in the U.S. On the other hand, Gefell customer service is incredible and they were able to take me through every step of the process, including locating the proper shipping method to send the capsule in and return it to Mali, which is where I was posted at the time.
Another factor is whether you plan to be taping for the long haul or if you are just getting your feet wet. I decided from the beginning that this was a hobby that I was going to pursue for a long time so I went ahead and got the more expensive option instead of flipping multiple sets of microphones. If you are unsure of whether you want to do this for a long time, go with the cheaper option and see if you enjoy it. It's the process and the music that I enjoy more than the gear anyway.
Finally, keep in mind that location is pretty much everything in taping. I have some recordings made with CSBs that blow away a lot of my other tapes with my big rig. The reason for this is location, positioning, and luck. The gear really doesn't have much to do with anything after a certain point.
John: How was the Gefell 85th celebration? I received an invite but could not make it. I need to plan a pilgrimage up there one day.