we use overlay majority of the time.
When you attach, your layer dialog box can become huge depending on how many other reference files are brought in.
Small jobs you could use attach, and attach could save you some time depending on how you set up your files.
example: if in your sheet file A-101, you have the arch base, mech base, structural base, electrical base referenced in.
But your arch base also have those same files referenced in. so if you used attached, you wouldn't have to reference the
same files in again...
Pieter