I've used the silicone (Dot 5) for 16 years now. Works fine, doesn't evaporate and the wheel cylinders don't stick as much. Vern Hunt recommended it to us years ago. However, I have heard others tell horror stories about it! Anyway, in AZ where you are, the temperature changes aren't as extreme and the humidity is very low so cylinders corroding may be a non-issue there as it is with my Dad's J (in Tucson). Here in lovely MN where there are huge temp swings and humidity swings, stuck cylinders are a big problem, especially with cars that are not driven very often.
The Dot 3 is a lot less expensive too so if you use it, I'd say do so. The thing you have to be careful about is that in a ventilated system, which was used in the older cars, KFs included, is that the fluid evaporates over time so you have to check the levels and keep filling it. Not really a big deal in my opinion but on a J, the master cylinder is not easy to get at to fill. I think you have one of those Darwin remote fillers correct? My '52 Vagabond has one and that works really slick but my other two cars don't have a remote filler so what I did was put a very thin layer of grease on the threads of the master cylinder cap, the put it on only finger-tight. This makes it much easier to fill the cylinder when I need to.
