The Government Center HI Select is a nice place, and the Mediterranean restaurant across the street is very nice for a quick bite to eat if you don't have time to go elsewhere.

If work is taking you downtown and the GovtCtr site is too expensive, then I'd stay at the Brookline HI instead of the Somerville one every time.

Avoid the HI Expresses. Some of them are converted HoJos, and the Cambridge one is in an equally uninteresting and inconvenient location as the Somerville HI Select.