The J seats in BR J and AC J are pretty much identical.
As you point out, BR J is generally excellent, though I personally wasn't blown away by my YYZ-TPE flight (terrible departure time, which in turn resulted in a rather rushed and disorganized service).
You can access the AC MLL with a BR J ticket, however the MLL closes around 10.30p.m. BR has an arrangement with the Plaza Premium Lounge for pax to use that lounge.