They're such very different environments that environment would be my first consideration. Lancaster's got a better rating for History according to league tables. Bristol still asks for higher entry requirements probably because the university will get lots of applicants - the university is old, pretty, generally well respected overall, and the area's regarded as trendy. Campus universities like Lancaster can be depressing for the highly sensitive or the highly social alike because they can be physically and emotionally cut off from the character of their actual city.
For 3+ years, a university will only be as good as it and its people treat you emotionally and intellectually. And that will spur you to work hard enough.