TsumegoFromGames51/Solution

Weak defense  

is a strong attack. If White blocks, is decisive (If at , Black plays at a).

Local Ko  

White should expand the eyespace. Black may hit the vital point with and make ko locally. Note that Black can destroy the upper eye by a throw-in at a. However, is a problem for Black depending on outside situation (White gets many local ko threats at least).

In practice  

So in practice, strengthening the outside in sente is preferable in most cases.