The car theft stats you can find various places, including at CityRating.com. In 2003, Somerville had 521 reports of car theft, versus Cambridge's 410 reports of car theft: when you capitate it, that puts Somerville's rate much, much higher, considering it has only three-quarters the population of Cambridge.

As to bike theft, I didn't have any stats until I read djdreilinger's piece! I just had that impression from talking with police in both communities.