On a related note I am about to book an AMS-LHR-MIA-SJO on AA.com.
On the AMS-LHR, AA.com is offering either the BA flight (BA423) or the AA codeshare on the same plane (AA6434).
On the LHR-MIA the booking is offering me AA metal (AA39) or BA metal (AA6162).
On the way back it is also offering me the choice of AA metal or BA metal (on an AA codeshare).
Generally speaking, which of these options is best for ensuring no check in glitches such as the above?