I would take Federer's first serve (if I had his forehand).
His first serve takes very little energy from him, and is very soft on the body, and it's very accurate. It's better for him to hit a second shot ace (forehand winner off short reply) than waste energy to try and hit first shot aces (what is counted as an ace).
BTW, Federer leads the ATP (last couple of years) in aces (if you count second shot aces too
Neways, the point I'm trying to make is it's better to have a serve that is soft on the body and doesn't take much effort. So for the kids out there, work on your serving form and accuracy. Get a consistent ball toss, and have good variety on your serve. Then back it up with a great forehand (give this more attention than your serve). And learn to his second shot aces... that is, learn to put away short replies.