A lot depends:

Abravise blasting is usually the fastest and cheapest. This fact depends on how strict are the local enviromental regulations on blasting in the open.

If you have to make a big containment effort the cost factor will quickly swing the other way. Depending on local environmental regulations and the intended end use of the steel there are additives that will help mitigate flash rusting.
 
Watch your contractor, if the regulations are similar to the ones in my state the cost of blasting in the open falls on the owner of the equipment being cleaned. The contractor gets the small fine and you get the large fine.

Agree with unclesyd. Big health issues with dry blasting, especially with sandblasting due to silicosis.
You may have to contain the blasted-off paint, if contains lead or chrome.

Adding about 0.5 wt% of a polyphosphate to the water prevents flash rusting (1% worsens paint adhesion). There are probably better but more expensive additives, and always environmental issues.