Short ride 20mi long ride 50 miles......is that short
I already own the phone but this would be the breakdown:
Cat eye double $85
Two sets GP4000s $170
4 tubes $28
Puma heart rate monitor watch w/monitor $60
iPhone with contract $250 unlimited data, my iPhone holds charge for at least a 50 mile ride with 65% charge left....
Total cost $593 and I use the phone everyday along with the watch on runs etc....
Why are you looking at the 800? I'd go with the 500 (or the 510 now).
Your overall question: Garmin vs iPhone (plus a myriad of other devices) is missing what I think are several important use cases.
First off, the Garmin's GPS radio is much more accurate and the battery life issue is an important one (as a result). I always bring my iPhone 5, but the GPS mode eats battery and out in the middle of a remote mountain road I may get cell coverage and having a phone that works if something bad happens is fairly important to me. Even on shortish "fun" rides on a bike without the Garmin, a 30 mile two hour "cruise" will sap the battery. Even on the 5 with push turned off.
Secondly, for my use, I train 6-7 days a week and I'd hate to have to spend time everyday fiddling with multiple devices to correlate training data. I can use Garmin Connect and Strava quickly: plug it in, upload the data, bam! HR, power, cadence, GPS, etc. No need to muck about.
Third, and this is a much more practical concern: I can see all my data real time. True, the 500 doesn't have map data, but I can look at vertical feet, distance, cadence, HR, power, at a glance. I can set up a few screens to fly through: when climbing I can see ascension speed, when on the trainer I can ignore distance and focus on HR, time, etc. This is huge. I don't want to have to try and fiddle with three different displays, that's just piss poor UX and I think it would adversely affect my training (who wants to be trying to push into Zone 5 and worrying about flipping between two displays?).
If you are looking for Map data and you're hellbent on getting hung up on retail prices, I don't see why you aren't looking at the 810 which can be had for the same price with the HRM and speed/cadence sensor.
Oh, extra bonus: less wires and junk about your person. What do you want to do, have mounts for your HRM, Cateye, and a funky case/mount for your iPhone on your handlebars? Throw in a few mirrors and some clip on aero-bars while you're at it...
(Secret confession: I do listen to music/audiobooks/podcasts on some solo training rides and that plus GPS will kill my phone in no time.)