If you fly paid business class and like to upgrade with miles, I would consider LH M&M. You earn 100% EQM bonus for C and 200% EQM bonus for F, compared to 25%/50% with UA's and AC's program. Furthermore, LH participates in the *A upgrade scheme, allowing you to upgrade with miles on several other *A carriers (details: here), UA has pulled out of that scheme and AC does not participate as AC only uses upgrade certificates good for their own flights, you can not upgrade at all using miles with AC's Aeroplan program. UA also limits the availability to redeem award tickets on other *A carriers (search for "Starnet" in the UA forum) and if you happen to fly within the USA, you can not access the lounge even if you are a Premier Executive or Premier Exec 1K, however Gold status with any program (exept US Airways) will get you access. Just my € 0.02.