Both the 3M and the 3S have 8 energy slots, but the 3S also has a missile slot. Normally in scenarios like this there are still differences, like in the case of the Stalkers and the number of missile tubes, making one a better LRM boat vs SRM boat, for example. But there are no missile tubes for the 3M soo... where is the benefit to using it?
I'm not seeing much of a point for the 3M. It can take a larger engine, but many will not be taking an XL400 in this mech. The only benefit of the 3M over the 3S is evenly distributed hardpoints. You can loose a torso without loosing all of your weapons. Same can be said about the 3S though, but on the 3S you will lose either energy+missile or your ballistic. On the 3M you will lose ~half you energy firepower. It's a bit more dependable because you can always count on your other torso. The 3M is really just a bigger Battlemaster/Stalker for energy builds.
While it does invite comparison to the Battlemasters and Stalkers, it reminds me somewhat more of the 8Q Awesome. I was able to run that model with satisfying results (at least to me...lol), so I'm definitely looking forward to slotting the same loadout into this chassis (with a few minor tweaks) and seeing how it does. As far as the 3S, it does have the ballistic and missile points over the 3M, but I am somewhat surprised that it does not have any energy hardpoints on the right arm, where the original tabletop variant had at least one medium laser if I remember correctly.
Distribution of hardpoints, and larger engine size. Run 6 MLas and some kind of LLas (or maybe PPC), and a STD 350-375.
The 3M is unfortunately not a frontline mech due to the lack of ballistics. It's a second line support mech for surgically removing enemies from the field by packing lasers. Or support fire using PPCs. Battlemasters are actually superior to the 3M due to the presence of 2-3 ballistics slots
Best thing about BNC-3M is that as the 3rd mech of the series you can buy this with cbills rather than getting LM. That is about it. BNC-3S has 8 energy points also, PLUS 1 ballistic and 1 missile. The only cons is a lower max energy rating (max345, so no STD350), but otherwise superior.
I prefer DHS. That, and a 95 ton mech build performing like a common heavy is not worth it. or even medium.