We've recently bought the Tractive GPS for our lab and we're really pleased with it. The tracker itself is quite big though, not sure how well it would sit on the collar of a smaller dog. It's not heavy though and is waterproof.
DH and I have both linked our phones to the tracker. You can set a "safe zone" of 50m, if the dog moves outside of the zone you get a notification on your phone. The tracker updates every 5 minutes unless you turn it "live", when it goes instant.
It's very accurate. I've had the tracker in my bag before now and DH has been able to watch me go up and down the aisle in sainsburys. DH often takes ddog to work with him so I can see exactly where he is, very handy when his phone is flat!
The only down side is we can't find a way to select a safe zone based on current location. So if you go on holiday and want to change to safe zone you have to scroll around the map of the UK and then zoom in. It doesn't bother me that much bit DH finds it hard as he doesn't know where most places are so struggles to find them.