I don't follow these multirotor forums too much, but occasionally pop in, especially when I'm working on a multirotor build. I've done quite a bit of reading on FCs but not much mentioned on any big differences on Tricopter FC's.

I've flown 3 quads with different FCs. A tuned Multiwii, a stock KK2, and a Naza. I found very little difference between the three. I think the Naza was a bit more stable in wind and high-speed maneuvers but that might have been a placebo effect. I've built a quad for a friend, and due to the lack of perceived difference, I went with a Crius Multiwii board, and was happy with the result.

I'm now working on a tricopter, and wondering if there's really any difference between control on, say, Arducopter on a APM vs. a KK2.

I know there are many more factors than simple flight performance, and I know a large part is personal preference, but I'm just curious to see what the community has to say on the matter.

The KK2 only supports two flight modes: Acro & Alt-Hold

The MultiWii & Naza boards support many more flight modes and also have the abillity to expand with GPS.

As far as the flight characteristics, that all depends on how well you tune your flight controller. A perfectly tuned flight controller will fly... well... perfect. An untuned board will fly like crap.

Naza's are by far the easiest to tune. KK2 a close 2nd.

Also one thing to note, if you are building a Tricopter you can count out the Naza as it does not support Tricopters. Only Quad, Hex & Octo's.

ArduCopter 2.9 is in beta testing and is supposed to rival the Naza in performance. Right now MultiWii & ArduCopter need some work on things like Altitude Hold. The Naza outperforms anything on the market right now as far as Altitude hold & position hold is concerned. But there are some improvements in Firmware comming that will give the NAZA a run for its money.