Mint. 15GB for $20/mo works out to $1.33/GB while your 2GB plan is $3/GB.
But there are other MVNOs out there like tello which also have a 2GB/$6 plan in the US, and other MVNOs which offer unlimited data for like $25-30/mo like visible and US Cellular.
Tello is actually what I use for my secondary data, Fi is my main (mostly because I travel somewhat and the data costs the same in all the destinations I care about without having to juggle SIM cards).
I'm not a good case study because I rarely use more than 2gb in a month, so Mint would come closer to $10 a gig... :)
