I'm not an older rider, nor do I know your riding objectives, so I won't comment on a power meter is a good idea or not. That being said, I've got experience with both Garmin Rally XC100(single sided SPD pedals) and the Favero Assiomo Duo Shi(dual sided with Shimano road pedal bodies.)

Knowing what I know now, the Faveros are a much better product, and at a better value IMO. One of the bearings disintegrated on my Garmin pedals recently, and the other side wasn't far behind it. Luckily I was just inside the 2 year warranty, so Garmin exchanged a brand new set of pedals. The customer service was excellent, but if this happened outside of the warranty, the bearings are not replaceable. Meaning that you have to buy complete pedal bodies(at $200 a side) just to replace the bearings. Another negative for them, has been the accuracy. To get accurate readings, I have to install them exactly at 34nm(which is a lot,) using a crow's foot and a torque wrench. Followed by a zero offset, a calibration, a few sprint efforts, and 1 final calibration. Otherwise, they are a random number generator...not exactly something that you want to switch between bikes all that often.

In comparison, the Faveros are pretty easy to swap between bikes. I install them like any non-power pedal(no torque wrench,) just do a calibration and they're good to go. That being said, I did end up with some sloppy bearings on mine, but I do put quite a bit of miles on them. Luckily, the bearings are replaceable...much better value prospect compared to the Garmins. The other thing, is the Q-factor is greatly increased on the Duo Shis...might be a problem for some, but it actually leads to a better fit in my case.