I've tried both (GPS and Iphone) and I have to say I LOVE my Iphone with the paid app. It seems to be more accurate, has less trouble under tree cover, and shows me better sat maps.
I had trouble with battery too, so instead of buying a $300 GPS unit, I bought this charging case and it's made of amazing for recharging in the field.
The paid app is WELL worth the money, IMHO. I also use the Perfect Mark app for puzzle caches.