I have never owned them, but I have had some degree of experience with both phones.
Specs wise, the ZTE force is the powerhouse of Boost Mobile right now. In terms of performance though, I'd say it feels neck and neck between the Galaxy S2 and the One SV. Both are extremely smooth phones for prepaid, but I have to give the trophy to the One SV. Integrated beats audio, nice tapered design, and a more aesthetically pleasing UI (IMO) make the SV the best phone on Boost in my eyes.
Plus, at $30 less than the GS2 with an LTE radio that the GS2 lacks, the only reason not to get the One SV is if you know someone with some old Epic 4G Touch accessories that you can have.