Hi Steve,
I'm running the PrInSu full length on my 2015 TEP and love it! I looked at the FrontRunner and K9 as well, when I was looking at racks. All 3 are great in my opinion. I was looking for something flexible that I could reconfigure easily as needs changed. Weight and aesthetics were also considerations, as was cost.
I've seen the FrontRunner racks in person on 5th Gens and have to say, I'm very impressed. I haven't seen a K9 in person yet. Both FrontRunner and K9 offer a lot more accessory options than PrInSu (i.e. mounts for MaxTrax, etc.) I think that will change in the future, though. Zach has recently released a new awning mount, which I will be installing soon (just waiting on an awning). I do have some concerns that the PrInSu awning mount will interfere with a RTT, but I'm sure I can find a workaround for that. Zach is also working on a hi-lift mount.
I'll second what
@vicali said about the modularity of the PrInSu. I can take the load bars out to clear my sunroof when I want. The T-slots make it VERY easy to attach pretty much anything. I've already made my own MaxTrax mounts, adapted a Hi-Lift bumper mount. While I don't like the idea of storing a hi-lift on my roof, I don't have any other place to put it for now. I love that all of the parts are replaceable and very reasonably priced. The fairing on the front can be replaced for one with a cut out for a 30" or 40" to allow for an LED light bar.
If you plan on running a RTT, as I will be, I've heard that bolting one onto the PrInSu can be a bit of a pain. I've read that some folks leave 2 of their load bars attached to the tent, making it easier to remove and reinstall. I think that'll be the approach I'll take, as I don't want to leave my tent on the truck all of the time.
On drilling: I believe the FrontRunner requires drilling 4 holes in the roof. The PrInSu is supposed to require 2 holes, and plusnuts with standoffs to attach the front 2 mounts. I didn't have the courage to drill into my brand new 4Runner's roof, so I am trying a workaround. From what I've heard/read, Gobi uses rubber feet that rest on the rubber weatherstrip. I happened to have some approx. 1.5" diameter x 1" tall rubber feet (pic below) lying around in my garage.
I bolted those onto the front tabs and it turns out they were just the right size. There won't be a lot of weight on the front of my rack, so I think this approach will work. I've been running my rack this way since the summer. However, rubber under compression will take a set over time. I've already observed some compression set in the feet and if it gets too severe, the rack could contact the roof. I may need to replace them from time to time, but worst case, I'll get over my fear and drill the roof.
All in all, I'm very happy with my PrInSu and would buy it over again, if I needed to. It's a great rack, very clean design and I agree that Zach Scott is a pleasure to deal with. He's a small business too, which I like to support. Hope this helps. Let me know if you have any other questions.