I can only really speak to the KC-135:

Pros: Its fast. Lots of hands-on flying. State of the art avionics suite (Block 40). Shorter deployments than other airframes (especially for Guard/Reserve units). Comparitively easy training requirements, i.e. no airdrop, no low level, no assault, no NVG, etc. There are still beans to count, but its not nearly as bad as the airlift bubbas.

Cons: Its OLD. A 50 yr old airframe will always have unique maintanence and reliability issues. Also, all of those state of the art avionics are not very well integrated into the airplane. You can datalink your way across the pond, but sorry, no autothrottles. You'll be dealing with 50 years worth of regulations. Ever hear of a "Non Critical Engine Failure Speed"? You will. There are things that made sense when we had little tiny engines hanging on the wings and were trying to launch at max gross weight ahead of inbound nukes that don't make sense today. Fun.

Overall, the tanker is a good, honest airplane to fly, and the mission can be very rewarding and a lot of fun. Of course, it helps if you like to turn left for hours at a stretch at FL250. If I had to choose between C-5, C-130, C-17, and KC-135 units, I'd go with the tanker. The people are good and the atmosphere is relatively relaxed. Plus you can go pretty much anywhere if your current unit is BRAC'd. Really, you can't go wrong with the Guard though. I've met very few Guard pilots (in any airframe) that weren't happy with their unit.