I'd recommend that you look into two loans rather than paying PMI. On our recent home purchase, we took out a primary conforming mortgage (75%), a HELOC (10%), and put down 15%. Thus we skipped the PMI, which would have added substantially to the costs.
We could have put down more and skipped the HELOC, but it didn't make sense for us. After the mortgage interest tax deduction, which we benefit from (not everyone does), the HELOC is well under 3%. The Fed's stance on rates make it seem likely that the rate won't be going up anytime soon, either.
The danger here with the HELOC is that the floating rate can turn around and bite you if interest rates suddenly go up. But if you have other assets, or a high savings rate and can expect to pay that money back in the next couple of years, it might be something you want to look into.