Mainland. But I came from Bukit Mertajam so my comment is bias. Lol
I'm seeing the potential of mainland rising up, as seen in Alma. The housing price now actually exceeded what I thought it should be. But then again, I don't really think BM is a place for housing investments. It's much of a place for own stay rather than investment to push the price beyond it's potential.
Overall I would say Penang is more of an own stay market rather than an investor market. (p/s: I don't want it to be so pricey beyond affordability. Keep the price at the right value please.)