My new eTrex arrived in the last week or so, and as the ordered rechargable batteries haven't yet arrived I stuck a couple of Duracell batteries in and took that and my old GT-31 out with me for a ramble yesterday (or pub crawl - we were marking CAMRA's Cider month by walking between 5 cider pubs over a 7.2 mile route). I wanted to compare the traces from the two.
So, this morning I loaded all the traces into JOSM, coloured the one set red, the other set blue and added the Bing imagery background. (Aside: at some point I'll probably upload these traces, though I tend to only upload in date order and often only catch up at Christmas; I've captured a few images and put them on Flickr.)
Both GPSes were in the same pocket of my fleece jacket, except when I brought out the Garmin to show how OSM maps were displayed and included the POIs we were passing. Thanks to Talkytoaster for the easily added OSM map. I've yet to try other OSM maps, though I believe others are available.
Based on yesterday's single test, in summary the GT-31 seems more accurate when I'm travelling on a bus or in a car (especially in the car). When walking across fields, the Garmin looks to generally be a little more accurate, though there was very little in it. The GT-31 creates a larger point cloud when in pubs (the Garmin was more likely to report a signal lost). The GT-31 was only set up to capture when moving above 1km/h (iirc) so the Garmin showed a dense point cloud when I was waiting for the bus home for 5 minutes that the GT-31 didn't.