Taylor is the manufacturer of the kit and tests. The TF100 is a repackage of Taylor components with bottle sizes that are suited for use in the TFP Method. If you buy a K2006C, you'll likely end up needing more CYA reagent, DPD Powder, and R-0871 before too long.
But for me, I didn't care that I had to buy any more of that stuff later. The cost difference was minimal and I like having the Taylor box and kit. Mostly that is because I've been using a Taylor test kit of some kind for the last 20+ years.
I also agree with the Speedstir being a great addition. And the sample sizer. I absolutely love being able to grab a water sample and not have to eyeball the water level, and then go to my testing table and let the speedstir do all the stirring. I have to admit though, I didn't buy either of those things initially. I went without at first to save the cash, but quickly ordered them anyway.