You should probably first consider whether your sampling is going to be destructive or non-destructive. That will narrow down the options greatly. Let us know and im confident we can come up with an appropriate solution.
Well, taking into account that I would work on more sites, I would probably go for a non-destructive sampling or, maybe, for a protocol that would take the destructive part as lower as possible (e.g. for getting info on the ratio between surface and biomass in a given subset of species or growth forms).