I would suggest splitting down the middle: put $1500 towards PMI and an extra $1k toward the SL. That may not be entirely mathematically optimal, but might work well if you want to hedge your bets a bit.
Personally, I also have a variable interest rate loan, but the balance is only $9k. I have plenty in savings to pay it off completely if the rate rises to 5%, but otherwise I route all of my cash flow to other priorities and am just paying the minimums while it is under 4%. If I did pay it off in one swoop, it would put a dent in our beater-car replacement fund, but that would be replenished soon enough. That savings account is my safety net when (not if) rates rise, but I'm taking advantage of my temporarily low rate to focus on other things.
However, $18k is quite a chunk of change to be nervous about the rate rising, and would take a huge chunk out of any cash savings you had to clear it out, so I would want to bring the principle down to where your cushion could absorb it if necessary.