First of all, a massive congratulations on your offers!
As a general answer, I would say that there is unlikely to be a significant difference between the type and level of work done in the energy/utilities/decarbonization space between BCG and McKinsey. They are both very large top-tier consulting firms that have deep expertise across a very wide range of industries, and energy/decarbonization is certainly a priority area for them both. At a high level, you should be able to find interesting work in this space at both BCG and McKinsey.
That being said, there are undoubtedly going to be some differences between the specifics of the work that each firm does in the sector. One thing to consider is the clients they work with. There may well be a good degree of overlap here, but there will also be companies that have standing long-term relationships with just one of those firms, so it might be worth asking about who their long-term clients in the space are. Another thing to consider is whether there are any particular sub-fields or technologies you are most interested in in the energy space. If you have more specific interests here, it might be worth seeing the extent to which each firm does business in that specific sub-sector. Finally, office location/staffing model is something to think about, especially given the COVID work from home situation. Many of the larger energy clients will be headquartered outside of Boston, so if there is a particular client you are interested in working with in a different city, would you be able to do that under their current staffing model, or would you be more tied to your home office?
I certainly agree that the best way of doing due diligence on this would be to talk to people at each firm. I would encourage you to talk to partners and experts specifically within the practice that deals with energy/utilities, and ask them a bit about their long-term clients and any current work they are doing. Staffing managers would be able to answer any questions about how the staffing model works.
Feel free to DM me if you want more details on any of this, and best of luck with your research!