Don't forget that the world got significantly 'smaller' during the 1500 years between Alexander and Genghis. So you can't really compare nothing, not territory, not time they needed to acquire. Fighting in different times against other empires makes every comparison a false approach. Interesting that someone mentioned the sustainability. I wonder (and I've been doing this for quite some years without finding a good answer, maybe because there is none available) how Alexander planned to rule/sustain his empire. There are to my best knowledge only fragments available of what should have happened in the long run. His biggest mistake was the way he appointed his successor, by not doing it himself.
On a side note to the barbarians: Sure, Genghis is a barbarian by definition since he is not speaking greek nor is he by any way *grai or *hell. Even if you expand the original definition of barbarians to fit more modern terms, he wouldn't fit.
That said, I'd love to have Alexander back with a conquering agenda and bonus that make you want to conquer. For example a boost to Science or Culture through conquering through Eurekas/Inspirations or having every foreign city under your control giving you a bonus of some sort to these things. For the Mongols, I'd vastly prefer Timur as a leader, even if some might find that out of place for Mongolia.