It's best to keep the pressure on the whole white group with . After the sequence to , black can attack on the top at 'a'.
here is nearly as good. The difference is that after , black can't afford to jump to immediately, but has to exchange - first. Compare the position of in this diagram with in the diagram above.
(Followup problem: what goes wrong if you do try at ?)
If here, then White can sacrifice two stones and take sente to play a big point next.