Oh and to muddy the waters I’m just about to get a used iphone 4 of a friend, would this do what I want the garmin to do?
Yeah, more or less. Get some mapping software (E.g. memory map) and perhaps a stem mount + waterproof case for it and it’ll do 90% of what the garmin 800 can do. The main issues:
1. Battery life – the iPhone will probably be ~4hours with the screen on all the time + GPS. Mine lasts 8+ hrs with Strava in my pocket (screen off). You can buy external batteries to extend this of course.
2. GPS accuracy – fine for determining where you are, but occasionally there are blips so the average / max speed /distance might not perfectly accurate. IME iPhone is 100% fine for route tracking and navigation but for perfect accuracy on Strava segments (if that’s you’re thing), the Garmin is probably better. The Garmin also has a barometric altimeter which makes height estimations a lot more accurate.
3. The iPhone needs extra attachments to use a heart rate monitor / power / cadence etc.