I have used both the Kirby Morgan Block and the RSV 1. The Kirby is reliable, but in terms of how we mount them (Clipped to a BC ring across our chest) we have a lot of excess clutter that seems to find every tree stump and snag. The Block connects to the primary and secondary tank (quick disconnect for donating the pony), includes the Drysuit inflator hose and the line to the FFM (EXO 26 or AGA).
Switching to a secondary tank is simple with the kirby, a good 8 full turns or so (alternate source starts to flow at 4 turns). With community team gear, I always check the kirby morgan position pre dive.
I prefer the Sartek block since it is simple to operate, and the hoses are not in my chest on the bottom. Pull the cotter pin and move the cyliner ram and you are switched. No one has accidentally switched that I am aware of with teams in my area. I am trying to get my hands on an extra RSV 1 from a team now. One of the local teams has been using them for a few years, trouble free.
I also like how easy it is to find the RSV 1 compared to the Kirby. Although clipped, there is some fuddling around to find it and get the knob. Timewise, my experiance is the RSV 1 is much faster for switching.
We use the Lifeguard Systems velcro harnesses for 19's and 30's. One dislike with them, unless you have dual bands on the BC, the velcro ring doesn't stay on 80's and the tank slides and knocks around. We pratice that the diver needing assistance undoes the velcro on the pony harness and takes the tank while the standby diver disconnects from the block and takes the pony reg off his necklace. I definately like the tiger idea that Bridge has recommended before... much easier. In the meantime, we live with what we have.