Thanks for the responses. For any existing OW members considering this, a health warning:

Although getting the comp'd status with UA is easy enough, retaining it may prove more of a challenge. Re-qualification for *A Gold is 50K paid miles [ie not including cabin bonuses] per membership year - this can't even be pulled off with a RTW, which puts a 39K miles limit on any single itinerary.

If I'd been able to look at retaining *A Gold with just one RTW, as is possible with QF, this might put things in an entirely different perspective.

On balance, I feel inclined to stick with OW.

Of the *A member airlines only Lufthansa and ANA appear to have a decent F product, compared with BA, CX, LA and QF in OW.

Route coverage between Australasia and Sth America is poor too, requiring a trip via Nth America [makes you appreciate the QF/LA codeshare between SYD-SCL even more for shaving 24 hours off the journey].