Welcome to the forum. Gramatically, both sentences are correct. "Decent" is an adjective and can be grammatically placed before a noun. Glass and claret are nouns. However, considering the definition of decent, if I had to chose one of these 2 as being more common, I would choose the first. It is a rather stiff, but is a sort of underhanded complement to a host for serving the corr
I'd say the common phrase would be a 'a decent glass of claret', meaning the claret is decent.
There are a lot more comments made about wine than comments made about the kind of glass it is served in. In addition, the kind of person who serves his guests claret is unlikely to give it to them in a water glass.