Companies like Boost, Virgin, and others are just leasing space off the main companies network. However there are terms in place where if there is network congestion their customers get the lowest priority. They also have issues with jumping to towers of competitors but that aren't considered roaming in certain markets.
It would probably be cheaper to do Sprint as an additional phone in the long run however it just boils down to commitment.
If I had to get a phone for a couple of months I would buy a used phone on CL and try the service as they don't give breaks on the phones.