To answer (kinda) the OP's question, Shinagawa has nearly non-existant action, but it's right on the Yamanote line, so easy enough to go up/down to Ikebukero, Shibuya, Shinjuku, Harajuku or across to Uguisadani.
Roppongi is the night life spot for foreigners.
Jesus. Are people still recommending that hole as a place for getting action? I don't think I've gotten any quality pickups there since the Lex was cool, maybe a couple here and there when Xanadu was still open in Azabu-juban.
I stay away from Roppongi (and Itaewon in Korea, for similar reasons) simply because it is the "foreigner area" and overrun with skanks and slags, which is all right if that's your thing.
Course, Roppongi Hills is a different story, but a little more higher class; gotta have money to get anywhere.
Train nampa and coffee shop nampa are still relatively easy for a freebie. If I strike out there, Shibuya clubs or in front of Ikebukero Tokyu Hands are the next stops on the free poon highway.