gigasnee44

Depends on who you take it to. AT&T for sure has off contract prices and will give them to you if you are off contract and request it (had a few friends go this route, not sure how much hassle they give you, though). T-mobile is all off contract pricing, so that is another option. I don't believe Sprint offers that option and not sure about Verizon.
 
As far as using Verizon as your service, it might be more difficult since Verizon is primarily a CDMA service and most unlocked phones are GSM (the OnePlus One is for sure). This means that most phones *will not* be compatible with Verizon, unfortunately.
 
I currently swap between a Lumia 925 and an HTC One M7 on AT&T in the NYC area. Since both phones have microSIMs (I think) I can just swap my SIM card back and forth as I need to. Obviously these devices aren't this year's high end phones but the idea is the same. As I mentioned before, my devices are both microSIM but you'll need to watch out for devices with varying SIM card sizes. In this case, you'll need a nanoSIM + adapter, likely. This experience should carry over to T-mobile as well since that service is GSM based. As far as I understand, Verizon and Sprint require you to call or take it to the store to deactivate one and activate the other.
 
If you already have an M8 and you're happy with it, I don't suggest investing in a second phone and swapping back and forth. I do it because I'm mobile tech obsessed and I like the varying experience and features on different platforms. If you feel like you want to dip a toe into it, I'd start with something even more inexpensive (I'm thinking Moto E/G or Lumia 530/635) and see if you enjoy it.