I have been a polar user (both bike and running - S725X with Footpod, Cadence and Speed sensors) for years. I like the protrainer software and data display.
Over Xmas I have used a Garmin Forerunner 410 on my right arm.

That garmin data for speed seems unusable for interpretation and very erratic. I also, through inexperience, used the virtual pacer but as Garmin measured short vice the race I thought I was on pace but actually was running 2 sec per mile slow. I started my polar 20 sec after start and because of the cycling on the garmin I rarely saw how long i'd been running so found running towards a time tricky. I manually lapped my splits on the polar and would do that on the garmin if I were to get one. Is this standard of data normal? Has anyone else used both? Is it just personal preference or is there more that I can get out of the Garmin. The HR functions appear very scant with the Polar Fit test, own optimizer etc beating anything the Garmin has. I have the S725X and am thinking of getting an old P1 GPS unit which works with it or going to Garmin as I like the ability to upload data to Strava and view routes. I cycle and dabble in tri so would be looking at the 310/910 although I am unsure as to how much i'd use the swimming functions and consider them to be gimmicky.