Hi @Emily Martin. Welcome to the Maine market! Whereabouts are you from?
We invest in rental property in the Portland area, but our vacation home is on Mount Desert Island (MDI). The two have some overlapping characteristics, but are ultimately two very different markets.
Briefly, Portland is more expensive but more accessible. Be aware that the Portland market is HOT. Prices are high, and many properties sell with multiple offers/bidding wars. Also note that the cities of Portland and South Portland (and other surrounding towns) are taking a hard look at short term rentals, and additional fees/regulations seem likely. That said, I've heard from STR investors that business is steaming along. Portland's reputation as a food/drink destination is making for strong year-round STR business (Maine is typically known for seasonality in this regard).
MDI is wonderful. We don't rent our house up there because we use it quite a bit. That said, if we chose to rent it, even for the season (which now comfortably extends from the end of May through the middle of October), we could make a killing. In fact, many year round residents are known to rent their homes in the summer and move to a camp on the lake to take advantage of the demand for STRs on the island. But... the locals basically roll up the sidewalks and shutter the storefronts right about this time of year. It is so, so quiet in the winter (I love that about it, but as far as rental income goes...). Some try to do 6-month rentals for the winter, but to mixed success.
Let me know if you have any specific questions about either area. I watch the real estate markets closely in both places. Good luck! I don't blame you for picking Maine!