By Scoutmaster I'm going to assume you meant regular Boy Scouts, and not Blue Loops or Green Shirts.
In that case you could go either way. I know in my troop the younger guys wouldn't quite ready for FTC yet however with the older boys the FLL ship has sailed.
If you did mean a Cub Scout Pack, FLL all the way.
As for Venturing, Obviously FTC, but honestly with funding I could see FRC. Unlike CS and BS, Venturing allows much more freedom with program and what one chooses to focus on. I could see a Venture Crews entire January February Program being based in FRC.
Either way good luck to that unit interested in starting a team. As a Scout myself, I can entertain the fact that many leadership and people skills I've learned in Scouts have come in mighty handy with FIRST.
Yours in FIRST and Scouting,