First, congratulations on both offers! You have a good problem to have.
Now, the first point I would make is: How could you not specialize? Any role you take is technically specializing! Don't get fussed up about this - you need to choose something out of college that will get you experience...the entire point of an MBA/MBB is to then zoom back out.
For reference, I joined an IT Consulting company out of college and specialized immensely. I had no problem getting into the school of my choice! The important thing was I rose up the ranks of my company and got a lot of exposure within my specialization (many clients, projects, industries, cities, etc.)
In terms of the 2 firms, I genuinely think you should pick the one that you feel will mentor/grow you the best? Which seems to have good people? Which has a good boss you'll work under? Which seems to be growing and doing good, high quality work? Pick that one!
All things equal, if it helps, I personally think Energy/Sustainability are fascinating spaces with a lot of current/future demand.