Terms for second aren't great from what I've been offered. If it is fixed rate, it is shorter, like up to 10 years. Otherwise it is ARM with a hefty balloon payment, often 1/2 of loan amount itself - which you can refinance at a cost. On the other hand, PMI is affected by good credit score; so it might be lower than you expect it to be...
Here's simplistic calculator to compare, in a long run 2nd mortgage will cost less, if you can get fixed rate with no balloon.
Second Mortgage or PMI Calculator - LendersMark.org