Had to pick Middleton. Dumart is easily at the bottom tier of the HHOF. Middleton is on many lists as being among the best to not be in the HHOF. Personally I maintained prior to 2005 that Neely wasn't even the most deserving former Bruin for the HHOF. That being said I'm not sure if Middleton belongs there but it's an interesting enough case.
Middleton was surely better in the playoffs. Both good defensively as been mentioned above. I think as a GM I would like the individual package that Middleton brings though, hard to pass up. Dumart is a guy that it would have been nice to see what else he'd have done without being shipped off to WWII. For those familiar with this site, there is a discussion players with a significant drop in postseason play and Dumart's name is all over it. Middleton's would never be.
Middleton's offensive results are better too, despite his lack of elite linemates and the team's defensive style throughout the 80s. Dumart, on the other hand, was probably the 3rd-most talented player on a very famous line, so determining how much of that line's offense he was responsible for is tricky.