Pancho
Where are you...Europe or US?
For Europe i would suggest material with a density between 650kg/m³ and 750kg/m³.
As far as i can tell the equivalent material in the US is Renshape5460 or 460.
The green material i used has 750kg/m³ density. The rule is simple...the higher the density the easier you achieve a nice surface and sharp edges....but it's abit harder to work with by hand. If you use machines definitely go for the higher density.
I hope that helps!