Originally Posted by
UAX_Brasilia
Both give Star Alliance Gold Lounge access. You can access any Star Gold lounge when travelling on any Star Gold airline, except for United Star Gold members who can only enter Red Carpet Clubs (which are Star Gold lounges) when travelling on an international itinerary.
This may be pointing out the obvious, but the lounge access thing is irrelevant if the OP doesn't fly enough to get mid-tier, which on UA is 50k miles in a year and for AC, I believe 35k. With once a year travel, this may not be the case. If the OP travels at least 25k though, UA may be preferable for the economy plus access on the international flight.
As far as earning miles, more or less you'll earn the same amount on either program (minus promotions). Be careful as some AC booking classes do not earn full miles. Other than that, redemption rates may differ, so take a look at the award charts for both programs and see which is most preferable for the awards you want to redeem.