I'm going to tell you my story, and you can decide if it applies to you or not.
I had a Nexus 4 with T-Mobile. I loved it, but one day it was lost. I bought a Note 3 from T-Mobile. Big screen. Fancy features. Really nice. My price for the phone was $800. That was added, interest free, into my T-Mobile bill. I paid $112 per month.
I found MMM and started to make changes to my life. But one thing I held onto was my T-Mobile service. By that point, I still owed $400 on the Note 3. So canceling service with T-Mobile would mean I had to pay off the phone immediately. The fancy phone, combined with the cost of leaving T-Mobile, didn't make it seem worth leaving. Even when faced with Republic Wireless, who's bill after taxes is around $30 or $50 per month for 3g/4g. So for the longest time, I resisted.
Finally, I had an epiphany. It was about the same time as my fancy phone's battery was dying (fancy phones eat batteries for breakfast, then want more by mid-afternoon). $30 per month means that I could pay for 4 months of service by the time I paid for one month at T-Mobile. Nothing about my phone was worth it. But one last thing held me back: My girlfriend's old crappy Android phone, a Motorola Droid Razr (1st Gen). Her phone sucked, in the same way it sounds like your phone sucks. I didn't want that.
But finally I decided to test out Republic. I even went with the Moto G, instead of the latest and greatest Moto X. That meant I only needed the 3g plan. The phone arrived, and I was prepared to ship it back with at "well I tried" attitude.
The thing is, the phone is snappy. I'm a Sys Admin, so I care a bit about technology. But the Moto G is fantastic. I can't imagine a phone that only cost $179 off contract working this well. The menu has no lag. There is no determinable difference between how well the apps run. I am satisfied with my G.
Now the one week side is the phone's camera. It's not the best in the world, but it's not bad either. If cameras are your thing, I hear the X does slightly better with them, so you might be justified in spending the $300 for that phone. But either way, 3g is fast enough to stream Youtube and Pandora without any lag for me. You can probably get the $25 ($30ish with taxes) plan and be just fine.
This phone is decent. It's beyond decent. And like you, years ago I had a crappy Net10 phone which sucked. It would run out of memory. It wouldn't install apps. GPS would tell me more about where I came from then where I was going. But Republic has the frugal market covered with an affordable phone which ROCKS.
So my vote is for RW. I know you're worried about coverage in the mountains. They have a generous return policy. Pick one up, and see if it works for you. Keep your current phone while you test the RW one out. If your old phone is still active, you can port that number right over to RW, so keeping it live makes sense, regardless if your test goes well or badly.
Cheers!