Well I got an Ergomo a while back - then proceeded to destroy my tri bike so my Ergomo was in limbo for a while (didn’t put it on the road bike…) but I used a powertap for a while in between…
Ergomo - Best points: any wheels, weatherproof, hard-wired, and I do not have to worry about wheel problem = loss of PT. Worst - no intervals and big computer. The main issue I have with the big computer is that it is hard to find a place to put it. The interval issue is not that big a deal if you use the cyclingpeaks software and a computer - you can find your intervals on the graph super easy - highlight them and get all your info. Any wheels is pretty obvious. Weatherproof - seems solid though I have not used it in nay bad weather but I wouldn’t be scared to (like PT). Hard wired = no dropped signals. Hard to swap to another bike unless you want to swap bottom brackets…The wires are fairly clean - one from bottom bracket to rear wheela nd one up to computer. If they made the computer smaller/sleeker and with intervals it would be close to perfect. Also Ergomo is rechargeable - no batteries to deal with. I have not had either serviced and not sure how that works…
Powertap - Best points: easy swap between bikes with 2nd wiring harness, nice sleek computer. Worst points: need to have dedicated rear wheel so you have to worry about trashing it if its a nice/race-like wheel (or its a heavier/sturdier wheel - or you buy multiple wheels…), and weather issues. Weather issues - I got nervous if it rained and I would not leave it in the car (on a hot day for a brick work-out as I was told that the heat can fry it though this may be a rumor). The wiring is a little more messy if you run the cadence wire as you have 2 wires - one from seat stay up and along top tube and another from chainstay along downtube. I dropped signals occasionally but never a problem. Intervals are cool and does make it easier to see on the computer (different shading), and if you travel without a computer, you can view interval data still. You can only view 3 things at once (which is plenty usually) and the display is quite customizable.
Which is better? I guess that all depends on how you want to use it. I did not ride either for more than a couple months so not sure how long/durable they are. I was happy with either and they both have pluses and minuses…I prefer the Ergomo on a tri bike as long as you have a place to mount the computer - especially if you want to run a disc or nice race wheel - or if you ride on nasty roads a lot and go through wheels…the powertap is nice if you want one powermeter on a couple bikes or to share with a buddy.
SRM - no idea.
Dave