Usually third party cell plans are cheaper because of limited roaming agreements. Post paid customers enjoy a much larger roaming area. Your phone may not say it's roaming when it actually is because of roaming agreements. If you stroll into US Cellular or Sprint areas with a prepaid phone there is no guarantee your phone will work on normal calls. 911 will always go through (usually).
I'm not familiar with phones that only make calls. My younger daughter does have an older Samsung phone only. It works great for her. She only gets Craigslist phones. I use her upgrade toward the smartphones.
I have the Samsung Galaxy S5. Battery life is has been great. My previous S3 would roar through the battery. The S5 can make it a full day on one charge. I'm a heavy user. It also has Super Saver mode. The S5 becomes a phone only in Black & White. Supposedly it can go 12 continuous days on a single charge. Camera with HDR is also a nice benefit.