I think they're very similar in surface things, like being shows with strong female leads who are also "butt-kickers", but I think it's also a lot more than that.
Both deal with a woman/girl who has a secret identity (slayer, spy.) Buffy and Sydney both have to navigate through a screwed-up world where everything seems crazy and out of control, and they both always feel like the fate of others rests on their shoulders. Both just want to have a normal life, but are stuck in completely abnormal circumstances. Both have a small but select group of friends who are aware of their circumstances and help in the fight, and Syd and Buffy both attempt to have some kind of love life even when everything seems to conspire against them. Both have been referred to as "the chosen one" on their respective shows; Buffy as the slayer, Syd as the subject of the prophecy, (and Buffy was also "Prophecy Girl").
Both shows are also similar in that they deal with the outlandish in a believable way. Of course, in "reality," there aren't vampires or genetic doubles, but both Alias and Buffy present their storylines with so much skill and depth and emotional realism that it allows you to completely believe in their crazy worlds.
I don't think it's just a coincidence that J.J. Abrams was a big fan of "Buffy", or that Joss Whedon is a big fan of Alias. The two creators appear to have a lot in common.