First, AA and B6 do not interline, so you cannot connect. Thus, two separate tickets. If you no show for your B6 flight, your tickets are cancelled and you deal with all of that mess. On the other hand, AA and UA do interline and if your AA inbound is delayed, it will be AA's responsibility to handle the reroute to SFO.

I would generally suggest doubling whatever it is that you consider sensible for a connection for a transfer between tickets. This allows for some measure of delays.

Second, if you like the B6 option, have you considered flying B6 GCM-JFK-SFO (even pricing MINT for the second segment)?