They're both the Slayer. 😛
Honestly though, we got to see a little of Faith's path to redemption, but she has alot to account for. My Judgement tells me that she's solid now, but there's no reason the scoobies should believe that. To tell the truth, it would be borderline insane for them to trust Faith fully after a week or so of her being back.
Does Buffy make bad decisions? Not even close to as bad a track record as Faith has. Giles may be more impartial, it's true, but his intuition has (from an historical standpoint) never been as finely tuned as Buffy's. He makes informed decisions, but she makes astounding intuitive leaps. It's a natural distinction too, seeing as how his training is very much to analyze the best course of action soberly, while her training tells her to think on her feet, and make snap decisions based on intuition and the moment. So I would have to say that Giles is the strategist, and Buffy the Tactitian.
However, Buffy's intuitions are not to be treated lightly on a strategic scale either. She sees things - sometimes by gut feelings, sometimes by an actual vision or dream, but I don't think anyone here can deny that precognition in one form or another is a Slayer's birthright; we have more than a dozen good examples of it in the series at the very least. So when it comes down to it I think that the group would be best served with Giles making the strategical decisions, but he MUST pay heed to his Slayer's intuition. If we are talking tactically, then I don't think it's an issue - Giles has been surrendering tactical control to Buffy since season 1, so I don't see a problem with her keeping it now.
So how does this apply to the latest episode? Well, Buffy seems to intuitively believe that the Brewery is an important spot for The First, so I am inclined to heed her judgement. Strategically though, Giles is right to preach caution. A better plan is needed than 'Just rush in there' but some kind of plan should have been considered - perhaps some kind of recon that posed less risk to the over-all mission. Maybe he might have chosen to abandon the idea after careful consideration. Buffy has definately been known for walking into traps, even when her gut instinct was right initially.
Summary: Giles with Strategic control, Buffy with Tactical control. Giles should pay attention to the gut instincts of Faith and Buffy.
On the other hand, I'm 99% sure that this latest power-upheavel will work out to their benefit, seeing as how they either win in the end, or Angel doesn't continue as a series. 🙂