I've been using GT-1000's for the past few years and always buy a full size up from my normal shoe size. Not sure how much mileage you are doing but I always find going up a size is more comfortable - especially if you are on your feet for a couple of hours or more when your feet are likely to swell a little. However even on shorter runs I don't notice any issue with the shoes being larger. If you've been happy with the GT-1000s then I'd be inclined to stick with them. You can usually find previous years models for £50-60 rather than going for the latest model.
I also have a 620 and have had no problems with both wi-fi upload at home and bluetooth upload via my phone. I didn't have to do anything special to get the wi-fi to connect on the first try. I have had the odd instance of the run not uploading and having to turn the watch off and on again but that happens rarely, otherwise it's been fine.
Depends on what features you need out of the watch really - I managed to get a second hand 620 recently for a little over £100 and have been very happy with it so far - had a few blips with the watch not mapping the correct route initially but that has settled down now. It's very quick to find a GPS signal, has a proper charging cradle and you can upload your runs over wi-fi which I find very handy. You just get home from your run and it automatically uploads, no need to have a computer on.
I was using the 410 before that (again bought second hand) but it was taking too long to get a GPS signal, the charging cradle was very unreliable and it would also be off course quite often as well.
+1 for vaseline - works perfectly for me. Both for the niplybobs and the happy sack -for some weird reason I get chafing down south depending on what shorts I wear so always blob a bit on down there for good measure.