As above, if you're moving fast (ie actually running all the way) you'll probably find that a handheld is a right old faff compared to a simple breadcrumb on a watch. Over distance on rough ground you'll have loads of time to use a handheld. I hate lugging mine around compared to the watch but can't deny the utility of a big screen with a decent map (see TalkyToaster). Either way, just make sure that you've uploaded your planned route before you set off and you'll be grand.
PS, Ignore ye olde worlde mappists unless you're especially interested in the discipline (and many are, which is fine) everyone else has moved on. While it's useful to be able to navigate in the dark, in a hurricane, with sideways sleet and peat bog up to your knees on Kinder with nothing more than a magnetised matchstick and an out of date OS while being chased by a bear, you'll actually be really glad that Ronald Reagan lobbed up some satellites, instead of being lost and trying to work out regional declination and whether that map squiggle is a rock outcrop or a marsh as your headtorch gets even dimmer and helicopter rescue actually seems like a viable way of getting home so long as you don't get the bill. Fallback if it's REALLY remote - have map/compass in the bottom of your pack in case of armageddon events and be able to find the nearest road if you can't hear it.
PPS phones are crap. They always stop working just when you need them.
PPPS a decent handheld is really expensive and has sod all use apart from the twice a year you'll not use your watch instead.
Post edited at 22:02