I'll pitch in my thoughts and opinion...
First thought is that I don't like SBRs shorter than about 9.5." A 16" carbine is, imo, a pretty handy set up. A 10.5" or so SBR with a 6" or so, light weight suppressor ends up a bit shorter than the 16" carbine with an A2 and maintains excellent balance. Just about perfect. So, I think a barrel length in the 9.5" - 11.5" range makes for an excellent start for an SBR.
I own an 10.5" and an 11.5" 5.56 and on occasion own a 9.5" - 10.5" 300BLK. I miss the 300BLK SBR or upper when I don't own one but I can live without... Not so regarding a 5.56 SBR, which I shoot a heck of a lot more for a lot of reasons, including ammo price.
A 300BLK paired with supersonic ammo doesn't do much for me, especially considering I own and prefer an 11.5" 6.8 SBR. However, as quiet as it is, a 300BLK shooting subsonic ammo always brings a smile to my face.
With the right ammo selection and a suppressor, I don't think there is much to choose between a supersonic 5.56 and supersonic 300BLK SBR for effective home defense.
So, as noted by other members, plan on acquiring both uppers eventually, but start with a 5.56. [But, if you reload, go with the 300 first.]
JPK