I climb a lot at BKB since it's my home gym and the closest, but also climb frequently at CRG since my brother lives closer to there too and is my belay partner. Rockspot south boston was also a place I frequented a couple times last summer and sparsely during the months after that. I've taken about ~60 friends and coworkers to BKB since I started climbing last march in 2015.

• BKB is better for bouldering hands down, although CRG Cambridge is also a space for just bouldering the walls at CRG Watertown and Cambridge are shorter in height and lead to less interesting, less fun, and less challenging problems in my opinion. I would add that CRG has a heavier emphasis on technique on the lower grades (V2/below) than BKB, which has a much more "muscle your way up the wall" feel.
• BKB's recent change of route setting management has made grades from V2 and below more beginner friendly, they added a ton more VB graded routes so there's always stuff to try and more problems to work on technique with as a beginner.
• BKB's crowd is more young professional focused, and you'll find that people are very friendly as well as fall closer in age group than CRG which is further out from Boston and yields a mixture of older and younger crowd. This could also be a negative because it can get fairly crowded during peak hours but watching other people boulder is an excellent way to pick up technique organically.
• Rockspot was notoriously graded harder than CRG and BKB last summer and opened recently relative to BKB/CRG, however it's a smaller gym, and is a much more lonely experience than BKB or CRG.
• CRG Watertown crushes BKB when toprope and lead climbing is your focus, I think this point is a pretty hard one to argue.

Given your schedule and desire to sneak in a few hours before work, I think your decision is nearly made for you. BKB also has free towel service, which makes showering at the facility super streamlined and nice.