We've done r-t-w trips using QF/BA and also AR/IB a couple of times in economy and once in Business. In Y think that there was a bit more space on QF than BA (if you check QF and BA seat plans QF seem yo have less rows in Y) suggest getting the two abreast seats at the back of the cabin - a little more space around you. Routing back from Sao Paulo - Madrid a couple of years back the IB flight was operated by Spanair (I think) - pretty poor service.
Last year got a great deal in Business using BA/QF (just over double the economy fare) - enjoyed so much that we've booked again for April 2002. E mail me if you need details.