I used an etrex20x for many years as my main brevet navigation before I got a bolt last season, and I still carry it as a back-up now that our club uses e-proof-of-passage. The etrex doesn't navigate you. It just shows a line on the map, so you can start and finish wherever you want or not follow the line at all. The etrex doesn't care, but it won't tell you when to turn so you may miss a few if you're not paying attention.
I also use a computer to transfer files to the etrex. The only reason I can think of to use a phone is if you didn't have a computer, which I do, so I have never considered using a phone. You may also need a computer to update the maps with
OSM. I use
OpenMapChest and have no idea whether or not there is a bittorrent client for an iphone.