First off, there doesn't seem to be much benefit in taking more than what's already present in a cup or so of good green tea, though, so the exact content in varieties probably doesn't matter.

If you ever get bulk l-theanine powder (it's quite cheap), you'll recognize the taste. It's particularly prominent in sencha, gyokuro, and other Japanese green teas - that savory/salty/MSGish taste.

Anecdotally, l-theanine negates most of the jitteriness from caffeine for me. Coffee, pop, etc. make me edgy in a way that tea doesn't.