Hello Ed here I've gotten out of 40k a few months ago, but i am more then happy to help.
I go to both stores, So I seen both first hand they are both great. Abington has two decent days Thur and Sat. Plainville has a great Thursday groups semi spear headed by Ben. The only problem I have with Plainville is there a lot of events on the weekend so the weekend groups are very static. If I were you I would just try one of the stores and hang out. People will be more then willing to talk 40k with you.