I've used both Garmin and an iPhone app extensively. For regular speed, distance, cadence etc... I don't think it matters much. You can get past battery drain with an extra battery or something like the RFLKT. The iPhone/smartphone has it all over the Garmin for display - not even close. Also the mapping on a phone tends to be better. Even when riding with my Garmin Edge 1000, if I get lost, I get out the phone to figure out what to do.

That said, Garmin or Wahoo have it all over the phone when it comes to calorie burn, power meter integration and other more performance based features. For example, the calorie burn in a ride in Cyclemeter (which I think is the best of the best of smartphone apps) is 2X what I actually see when using a powermeter and about 40% less than Garmin's calculated results. So that's a huge difference and worth noting. Truthfully, the calorie computation from anything other than a system with a power meter is almost worse than worthless.

So I think if you are just going to use it for general purpose ride metrics, then the phone is fine. Pair it up with the excellent BT capable sensors from Wahoo and you're in business. If you are after more performance data and accuracy in calorie burn, then the computers are better and plan on adding a power meter.

J.