I would think liquid is better. It is closer to its natural form and also a lot cheaper in the long run, if you look at its active ingredients. Plus, (usually) no fillers.
I would probably think that whatever you could get at a health food store will be higher quality, e.g. purer, but of course also more expensive.
I use the HerbaGreenTea from HerbaSway. I really like it.

Hope this helps!