Use whatever your buddies use! There aren't any overlaps (yet). Scala will only work with Scala, Interphone with Interphone, etc...
I have an Interphone F4. I love it. 12 hour battery life, the loudest/clearest speakers I've heard (including the F5), long communication distance when paired with another headset, and relatively cheap these days. My buddy has an F4 that his wife uses (used to be his) and an F5. The 5 has all sorts of nifty features, like AM/FM radio, multiple headset capability (I can talk with him AND his wife on rides due to the dual throughput on the F5; it acts as a hub, even though the F4 can only pair with one other headset) and an input jack if you don't want to waste your iPod's battery with Bluetooth.
"Definitely do not sit on a DL1000. Big mistake. You'll end up buying one. Happened to me. True story."
"Purchasing a V-Strom from a Harley dealer does have poetic justice. Almost like removing a talented child from a meth house."
"I'm for moves that will help people get over the idea that it is ever okay to drink and drive."