I have a 4iii single right side PM on my gravel bike (not enough clearance on the left side). If I am planning on putting a single side PM on my road bike the best “apple to apple” comparison would be to put a right side one on the road bike too, right? This seems like a no brainer to me but maybe I’m missing something…
Yes, if you have a power imbalance (pretty normal), then you would want both PMs measuring the same leg.
My imbalance is different on my road and TT bike. One option would be a Magene crank-based PM. $300ish dollars for a dual sided crankset with the option of different crank arm lengths.
I use a Stages power meter on my Road/TT/CX bikes. They’re all FSA BB386 EVO bottom brackets which gives me a threaded bottom bracket but a 30mm spindle. I love it.
It takes less than a minute to swap my power meter from one bike to the next. Sometimes I don’t bother putting it on my CX bike unless I’m going on a longer gravel ride. I use this bike as a commuter with single-side SPD pedals so I can ride without cycling shoes. If I’m doing a longer gravel grind then I just use my road shoes.
You could probably get a new power meter, or you can get a new crank/BB to match it between your bikes for less money (maybe).
If you want to be able to use one on one bike and one on another and hope for any consistency, I would definitely get two power meters that you could ride at the same time for a while to see how they match up. Power meters are not as accurate as we wish them to be and even two power meters of the same brand and probably don’t read exactly the same.