Neither if you have a choice - Metposts are not terribly good at
holding a fence no matter how you put them in.

Personally, I use concrete-in Metposts using the following order:

1. Dig hole 1 spade wide 1.5 spade deep.
2. Insert post into Metpost using sledge hammer.
3. Stand post in hole.
4. Adjust position.
5. Adjust plumb.
6. Buttress post to prevent movement. I frequently attach the fence panel at
this point to ensure position and plumb.
7. Pour in half a bag of post fix.
8. Water.
9. Pour in rest of post fix.
10. Water.
11. Don't touch for 24 hours.

The advantage of using this method is that you have the advantages of
concrete (longetivity and strength of fixing) coupled with the advantages of
metposts (wooden post not underground, can be replaced).

Christian.