Depends on your hauling situation what kind of haul line you need.
If you are climbing big walls, and hauling tons of stuff, you really should have a static rope because it makes life a lot easier, especially with mechanically advantageous systems. That being said, Kate's haul line on BUBS was dynamic, and it did work ok on a 2:1.
Haul lines take a ton of abuse, and skinny ones wear out too quickly. They are also scary to rappel if you are soloing. Better to go with a burly one, something around 10.5mm or even 11mm, as long as the line is flexible. Beware of stiff caving ropes that are made for jugging, not hauling - you want a nice flexible rope.
You should also buy a long one - 70m. This is handy for linking pitches, fixing pitches, and other big wall rope tricks. Plus the parts of the rope that gets abraded and nicked the most are its ends, and this is what you may want to cut. Actually, I never cut the end of my haul lines off, even when nicked, because I can always tie on above. I like to use my excess haul line as lower-out.
If your loads aren't that heavy, a retired dynamic rope is perfectly acceptable as a haul line. It can also double as a spare lead line should you damage your primary lead rope. If you are soloing, though, you don't need a spare lead rope, cuz if you damage it, you can put the damaged part in the middle of a butterfly, no biggie when soloing.
Cheers,
Pete