This depends on the methods you intend to use. Thermal methods, such as fast pyrolysis, would prefer dry material. Extraction methods will also prefer dry material if the solvents are hydrocarbon based. If they are aqueous or are miscible in water (such as alcohols), wet biomass will probably work as well or better than dry biomass.

I recommend you to use freeze-dried sample.

1-2 gm depending on your experiment