I am also considering a PowerTap and am wondering, for the purpose of multisport, why would one NOT want a Garmin wrist unit that you could use on the bike with your PowerTap, that you could then remove and wear for the run?
Using the 310 is not as accurate. The recording interval is less and not configurable. I’ve compared ride data between the 310 and LYC and power numbers are off. That doesn’t stop my from using the 310 for the run but I certainly don’t look at the power data.