Before you do anything, talk to your mortgage provider. Some loans require 22% down to pay off existing PMI. Afterward, here's my thinking:
I usually look at PMI as if you had a second mortgage. You have a $222k mortgage at 4.2% and a $30k mortgage at 4.2%+$120/month.
This looks like your PMI is costing $1440/year on $30k. That's about 4.8% on top of your current 4.2% mortgage rate (which costs about $1260 for the top $30k of your mortgage).