I've been using an Iphone on the Straight Talk network for a couple years now and have been very happy. I would never switch back. I full time and have been using it all over the country with no problems.
I recommend getting a phone that uses the Verizon network. That's what I did. They seem to have the best coverage. I also use the Straight Talk auto-refill every month.
Straight Talk is unlimited voice, text and Data. I think I get 2 gigs of data at high speed, then the rest at lower speed. So it really is technically 'unlimited'. I monitor my data usage and usually am right at 1 gig/month. Just don't stream movies, etc and it will be fine. I do not make any effort to minimize my data usage.
You cannot use it as a hotspot so it's actually pretty easy to stay under the 'high speed' data limit.
If you want to use the phone as a phone, do some texting, read mail, access various apps, use Google Maps and surf the web a little it's ideal. I suspect that's all many people really do with their phones anyway. So why pay those insanely high prices ?
Tim