Leominster is a bit better than Fitchburg. I think of Leominster as a firmly blue-collar town (with that one random neighborhood up in the hills with giant homes), and Fitchburg as being run down.

That being said, I lived in downtown Fitchburg for a year and it wasn't that bad. Definitely a seedy town, but seedy by Massachusetts standards. Not uncommon to get asked for money at night, but I never really felt in physical danger. Northen Fitchburg gets much more suburban if not downright rural and has some nice quiet neighborhoods (especially around the high school).

Tons of towns are Fitchburg would be a fine choice, think Lunenburg or Townsend maybe. Rent should be cheap, but as some advice from me to you? As a 20 something, unless you're deeply introverted, you will be in agony living in those sticks out there.