DELFT CLAY: By far the most well known sand (called clay due to how compact it gets) on the market and the favorite by many due to it's ability to hold it's form when removing objects that have been pressed into it. Super for detail but also very dense which I find can be a negative when working with fragile objects that may crush since the sand grain is so fine and compacts so hard. It has no give. Delft is also the most expensive of the other sand options but since oil based sands have a long shelf life this isn't too big of a deal. This sand also requires venting to remove air and gas build up when molten metal is poured. ***Good ventilation is a must!
PETROBOND: Very similar to Delft but a more course grain which makes it not compact as tightly and can break apart if a person isn't extra careful when removing an object. In detail tests between both Petrobond and Delft I haven't noticed a big difference overall but Delft is just easier to remove the objects. It is however, much easier to press or compact fragile objects into so in some ways I think it may be a great idea to have both if sand casting is a big part of a business. ***I also feel that Petrobond is best for the beginner since it requires no venting and is quite affordable. ***Good ventilation is a must!
My sand choice is to combine the two 50/50 which offers the best of both and works awesome!