Reliability of pacing with an iPhone 6S

lightng wrote:

I've been using the iSmoothRun app with my phone since I love listening to music/podcasts while running so even if I were to get a watch, my phone would still be somewhere on me.

I'm wondering if i'm sacrificing a lot by not having a dedicated GPS watch?

The last half marathon I ran the app recorded a finishing time of 1:49:25 whereas my official chip time was 1:49:58 so a differential of about 33 seconds (or 2.5 sec/mile).

Is this a normal margin of error that people with actual watches experience as well?

In order to deal with this should I adjust my average pace down by about ~ 2 sec/mile if I have a goal time that I want to beat?

I know that my Garmin 235 gives a different time/pace to my Samsung smartphone. I think the watches, or at least the more expensive ones, have more accurate gps than smartphones.