Invading the 4-4 6-3 enclosure.
Dieter: It's difficult to give an answer without knowing the whole board position. The presence of stones at the circled point can influence the situation in a major way. Anyhow plays on the inside, so a light approach seems called for.
Usually, Black will play at , since White can only live in ko and the outside here is open, which makes outward influence an interesting feature. When the outside is not open, Black may play like you suggest, killing White (if the outside is rather black) or preserving the corner (if the outside is rather white).
The - exchange before is kind of kikashi. Next is the only way to kill, but without more support here, Black will not be able to sustain such an all out attack. Locally, a-b-c and d@b follow.
Bill: As Dieter says, it is hard to say what is best without seeing the rest of the board. The invasion is bad on an empty board, so we expect to see White stones in the vicinity.
That said, my first thought is , and if , then . Black now has three disconnected groups. Not so easy for Black.