Since the two criteria the OP asked about are smarts and team cohesion, I will also have to give the edge to AI.
Smarts:
Fred has genius-level intellect, unmatched by anyone in the verse (I mean come on, she was living in a cave for five years and still managed to return and publish a physics paper that seemed to have been widely regarded in the field, enough to land her a speaking engagement at UCLA or something). Willow is the genius of the Scooby team, but this gets watered down significantly as she grows to rely more on her magic and less on her brains.
Between Giles and Wesley, I think it'd be a little unfair to compare intellect. We know that Wesley was at the top of his class in high school (head boy), and that he has access to the seemingly bottomless resources of W&H. But Giles is obviously a wise and experienced Watcher and a masterful researcher. I'd say these guys are evenly split.
And Gunn, even though he is often relegated to "muscle," we can't discredit his knowledge of the streets of L.A., and of his resourcefulness in fighting vampires before he joined AI. Although Gunn is never considered the heart, I would say he is more or less the AI counterpart to the Scoobies' Xander: the most expendable of the official team members.
I would say Lorne is something like Tara: both are peaceful, and both serve, more or less, as a sort of conscience for their respective teams. (I just had to mention this, although I think it adds little to the actual discussion at hand.)
Buffy and Angel are the leaders and main strategists of their respective teams. Here, I would also say that Angel probably gets the edge, largely because of his 2.5 centuries' worth of experience and because he has inherited all of Angelus's scheming genius, if not his sadism. Angel puts into motion several intricate, long-term strategies (e.g., the final plan to take down the Circle of Black Thorn). In contrast, Buffy's plans, while effective, tend to be more of the "play it by ear" type, just going in blind and punching things. And there are several occasions when we see that Buffy doesn't really think through the potential consequences of her plans (e.g., did nobody on her team think through the possible ramifications of activating every Potential in the world? Surely Willow could've limited her spell to just the ones who had come to Sunnydale).
Team cohesion:
While both teams work well together, I give the edge to AI because I think they just had a clearer allocation of duties. Even though BTVS had the whole "heart, brain, hand and spirit" thing as seen in "Restless," I think the roles on the AI team were just more clearly divided up. I think someone else mentioned this earlier in the thread, but they were in the business of fighting evil, so of course they would have had a better idea of each individual's specific role and responsibilities on the team.
