I don’t have a power meter on my outdoor bikes, only my Kickr. I know that Xert Online can derive power from HRM and cadence data, but are any of the Xert Garmin data fields usable if you have an HRM and speed and cadence sensors, but no power meter?
Remember the old saying knowledge is power? Reverse that. 
All the data fields require power – Connect IQ Store | Free Watch Faces and Apps | Garmin
Best to consider a power meter for the bike you ride the most. That will also improve HRDM for any bike you ride with HRM and cadence only.
Least expensive option will be a left crank model.
Most expensive will be pedal based but that allows you to move the pedals between bikes.
Thanks, I did see the “power required” note but thought I’d ask since XO can use derived power. Trying to get by with HRM and cadence sensors on outdoor rides for now, will get power meter (probably pedal based, and probably two sets for road and MTB/gravel) when I convince myself to spend the money!
I have the Favero Assioma Duo pedal system and it is easily interchangeable between bikes, no need for multiple power pedals. Also the Uno is cheaper than the duo if you want to save money.
I hear great things about the Assiomas, but I use 2 different cleat/pedal systems for road and off-road riding, so can’t use the same power pedals on all bikes. And I switch between road, gravel and MTB frequently, so the new Garmin pedals with changeable bodies aren’t practical either (they’re really only intended for occasional changes).