Sensei's Divining Top is far less like Brainstorm than it is like Mirri's Guile. The principle advantages of Brainstorm are that it digs three cards deep, hides quality spells from Duress, et al. and that it can help eliminate dead draws when combined with a shuffler, such as a fetch land.
Sensei's Divining Top only really allows you to select the best of your top three cards for the cost of
each turn. Although the ability is vaguely inferiour to Mirri's Guile, Sensei's Divining Top
does protect itself, by jumping back to the top of your library at instant speed.
However, my point is this: Sensei's Divining Top is really only comparable to a card that does not see play, rather than one that does. I very much doubt that it will replace Brainstorm, except perhaps in decks where consistent card quality is more important than the added functionality of Brainstorm.
