I started with iPhone but bought a garmin 510 and am pleased with the choice.
My iPhone 3gs battery life was short and I can't read it in sunlight so I needed another computer anyhow.
The iPhone shuts off at temps 105+. It does better turned off in a jersey pocket where it is out of the sun.
I had to put an external battery and a cable into a ziplock bag with the iPhone to get through a 35 mile ride when running a GPS app.
My iPhone is in a otter case that prevents me from using an Ant interface and it doesn't have bluetooth. So I couldn't integrate heartrate data or cadence, nor power if I choose to buy a powermeter.
I also used to accidentally discard rides with the iPhone strava app. There was a delay after touching the save/upload hotspot and I would accidentally tap it a second time which deleted the data. No problems with that on the garmin.
I do miss the ability to immediately upload data post-ride, and the need to use a computer and USB connection.