Where to hook a ski rope

is it better to hook a rope for slalom skiing on the wakeboard tower or to the stern of the boat

Definitely better to use the tower!

Slalom skiing is best done from the rear of the boat. The pull from the tower is too high for the "proper" pull when on a single. However, since you are asking... it probably means you don't do much slalom skiing... otherwise you would already know that answer from experience. In that case, it doesn't really matter. The benefit of the tower is that it can help a novice get out the water a little better.

I do a lot of slalom skiing but this is the first time I had a tower.

Well, try it both ways. If it's basically just fun/recreational type it won't really matter and you might like the "upward" pull to get you out of the hole. Might take a little longer since you'll pull the bow up a bit, though. If you start pulling hard outside the wake, and you're attached to the tower, you might be surprised how much you can lean the boat over on it's side!

But, if you think about it, there's a reason that pro's don't attach to high point. Wakeboarders... that's different... the high attachment point gets them the big air.