Hi Jeremy, some of those markets in Northern NH can be tricky for STRs, due to local restrictions. The seasonality of the NH market can make it tough to find the "perfect" area. Seeing as properties in the Lakes Region aren't super close to the White Mountains and vise versa. (Skiing in the winter, hiking in summer vs lake properties during the summer.)
Salem, NH is experiencing a lot of development and growth. Most of which is attributed to Tuscan Village, which is located in the heart of town. Real estate is very expensive in that area, so it might not make sense from a numbers standpoint. Many people living in Salem are doing so because they work in Massachusetts. Both Nashua and Manchester have a similar feel, although Manchester is generally more appealing. It's the most populated city in the state and offers a lot of jobs in various industries. It has more to offer for long term tenants, including multi-families, condos, townhouses and single families. (Although SFRs are usually the most difficult to find.
As for Concord, it's much more laid back and doesn't have as much opportunity as Manchester or Nashua. (For the younger working professional.) However it's centrally located, which makes it attractive for those into outdoor activities. (All of the state offices are located in Concord.) Any of these areas would be a good spot for long term rentals. With the exception of Salem, since it will likely be tough to find cash flowing properties.