Personally I'd say stick with MyBB. It offers you with more than the others and it's just easier. I have experience with both XenFero, VB, SMF, IPB etc. And The one I found the most appealing was MyBB mainly because it is supported so well. You already have everything set up with MyBB so I just don't the need for the hassle of switching everything over and trying to mimic the plug ins and features you have here on another forum software.
Most people that are long time users here came from "HeckForums" and they are already used to MyBB so the change might not be good with users that have no experience with the other forum software which in turn may lose you traffic. You have been on MyBB for so long, the drastic change is too much in my opinion. Don't fix what isn't broken, I love the site how it is. Just my two cents. Your decision though and I'd stick with FK either way but wanted to voice myself.