Honestly I think the Northants Blast side could beat the Phoenix.
One huge factor for stuff like this is how much or little time the players spend together. For crisp teams, it's eight games a season of a format that isn't played anywhere else and they come from all different counties where their roles often overlap (not to mention the disruption of England and overseas players coming and going); for county teams, they play 14 group games as well as usually about 10 second XI T20s with the core of the Blast side, and a group of players will often stay together for years at a time. The crisp teams may sometimes have better individuals (not the Phoenix) but more counties have better teams.