If you paid off the contract now, it would take about 19.5 months to "break even" (30-12 = 18. 350/18 = 19.4444). Since 19 mo > 12 mo (or however long you've got left), you're better off to finish out the contract.
To put it another way, if you have 12 months left at 30/mo, you'll pay 360 over that year. If you pay 350 now, AND 12/mo, in a year you would have paid 494.