Idan Y,
You are right about Android letting you use GPS while in Airplane mode. I still don't understand why the Iphone can't do it, even with a Jailbreak I wasn't able to find a way.
Androids are cheaper, but wouldn't recommend going too cheap. Worth paying a little more for the better ones.
As for Iphone vs. Android, it really does come down to what works best for you. Don't let price be a factor in something you are going to be using daily for at least 2 years.
Gaiagps is on Android and Iphone. Currently I think it's a little more developed on Iphone, but they are pretty much the same. Completely worth the $20. You can use Gaia Cloud sync to choose trails from the online map and then they'll automatically sync to your phone.
In terms of battery life that comparison isn't fair at all. You are comparing a phone with a 6000mha battery vs. one with a 1500mah batter. If the Iphone would had had a 6000mah battery it would have lasted almost 2 weeks.
Phones in terms of being weather resistant. Only a few number of Android phones are "Waterproof" I wouldn't put the GS5 under water. My friend has a Sony z3 and it's meant for that. There was an unboxing of it underwater, no special case needed.
Down side with the Iphone, you'll want a case for it. If you want water proof, don't get an off brand case, or knock off. It'll be rain proof and maybe dust proof, but don't expect them to keep your phone dry underwater. You shouldn't really trust cases like the Lifeproof to do that either. The bright side to the iphone is that because it's the same, companies can make a wide variety of cases for it, so you do have many to choose from.
Also, please don't let things like 2.6ghz quad core, 16 Mega Pixel camera, etc persuade you.
Just because the numbers are higher doesn't mean it's better. The iphone's camera might be only 8MP, but is one of the best if not the best Camera phone out currently. That was one of the main reasons I got mine. The video stabilization is insane. (I like taking video as I hike.)
I'll leave you this review to explain. One of the best reviews out there for it. Tons of video and picture comparisons vs. other phones as well with a quick Google search.
http://www.anandtech.com/show/8554/the-iphone-6-review/5
Edit: The iphone has a 1.4ghz dual core by the way. Apple just has way more control over the software and hardware so can get more performance out of a slower cpu.