Aldo.. More experience and the better athlete... Fight is 2 months away and I'd bet that someone gets hurt and ops out if you really want to know what I think...
Aldo has fought way better competition and he's a proven champion. Dude is durable and can handle going the distance for 5 rounds. Not questioning McGregor's conditioning but he has never fought through the 5th round (credit his finishes). My pick is Aldo by decision or late stoppage.
aldo is a black belt in muay thai, a bjj black belt (placing at worlds/pro's and a multiple brazilian national champ) under andre 'dede' pedeneiras and a luta livre black belt under marco ruas. conor is a brown belt under john kavanugh, and has zero grappling credentials to his name.
worth mentioning that it took conor 6-7 years to get that brown, eventhough he's been training daily.
aldo is a proven champion, he'll be the faster fighter, and he has a more diverse arsenal standing then conor who while an exellent boxer offers little else but a left hand and a bolo uppercut.
aldo has gone the distance multiple times, winning the fifth round both against edgar and mendes, conor hasn't been 25 in his life, he cuts more weight, and he has a style that is alot more demanding in terms of energy usage.
aldo will be my biggest bet of 2015 and probably one of my biggest bets ever.
ps: conor maybe younger then aldo by a couple of years, but it's still conor who's suffered the more debilitating injury having torn both his mcl and acl to shreds.