Depends on your application. If you were to run a single sub setup, it will be more efficient to have a Dual 2 ohm woofer to parallel the wire to see 1 ohm.
When getting a Dual 4 ohm woofer, it would be recommended to get 2 of them to also parellel and drop it down to 1 ohm. Basically, which is more efficient. Unless you have an amp that puts out a certain number of watts at 2 ohms, then that's when you would get one sub Dual 4, which then wires to 2 ohms.