Question on Insert / Update

Hi,
have just a general question...

I have a table of 10M records, unique key on 5 fields.
I need to update/insert 200k records in one go.

I could do a select to check for existence and then either insert or update.
Or simply insert, check on the error code an update if required.

The 2nd seems to be to logical choice, but will it actually be faster
and moreover is that the right way to do it?

Thanks
Alex