Dual 4 you can run 2ohm or 8 ohm (depending if you wire it in parallel of series)
Dual 2 ohm can run 1 ohm or 4 ohm (depending if you wire it in parallel of series)
I suggest you go to BDL or somewhere else that has the Type Rs and listen to all of them.
Basically, in a vehicle, you would not go higher than 4 ohm as running anything higher requires alot more power.
The basic principle with ohms is, a 4 ohm driver will require twice as much power to run as a 2 ohm subwoofer, however, you get much clearer sound quality.
If you've hear subs at 1 ohm, usually high SPL subs, they sound like garbage, but are hell loud.
You need to first decide on what you are after, SPL (1 or 2 ohm subwoofer) or sound quality (SQ) 4 ohm subwoofer.
if you choose 4 ohm, you will need a much more powerful amp to run it.
You will also notice with amps that some are 1 and 2 ohm stable, when running 1 ohm, the power output usually doubles, but sound quality decreases.
As I said, got to your local audio shop, listen to each sub, see what you like, then you can make a decision from there.