I've never been a fan of phablet phones (especially for running and excersize) but I'm going to make an exception in this case.
The main reason: Battery life. The Iphone 6 Plus had a substantial upgrade to the battery life over my current phone 5s.
During a run, I typically run several apps at once (Run tracking software such as Nike+ and music streaming). Not including web-surfing, that burns approx. 10% battery life per hour on my current Iphone 5s Phone. The Iphone 6 battery is not much of an aprovement over the Iphone 5s, so I could expect similar results.
On my current phone that equates to burning a minimum of 20-30% (or more) battery life during the race itself. Just ran a 15K last weekend and my phone died mid-race with no warning (from being fully charged at the start of the day. Lasting only about 4 hours). That was devastating.