One of the reasons I prefer using a phone for gps is that you can load any app onto it, you're not tied to "oh, your hardware doesn't support that".

If battery life is a concern you can buy an external battery pack like this one for $30:
Amazon.com: USB External Battery Pack 20000mAh 5V/2.1A (Black Limefuel Lite LP200X) Charger with Flashlight for iPad, Mini, iPhone 6, 6+, 5S, 5C, 5, 4S, Galaxy S5, S4, S3, Note 3, Nexus 4, 5, 7, 10, HTC One, One 2 (M8), Motorola Droid, LG G2, G3, MOT

Cell phone batteries are around 3,000mah usually, this pack is 20,000mah.

The only reason I would personally buy an external gps unit is if they had some magic that made the screen far more visible in daylight.