I don't know what your tastes are, but......
27th Man has some nice tunes (like Gregory is Here and Summer in Central Park) and is a enjoyable recording overall, but I wouldn't consider it as essential as some other Silver (I'm still waiting for the Silver and... series). I would, however, recommend 27th Man over the recent Silver 2fer reissue The United States of Mind.
The Prisoner is essential, however. It ranks among Herbie's top Blue Notes, IMHO. It represents a high level of artistic vision (compositions and arrangements) whereas the Silver is a little more "routine" in terms of the head-solos-head format, but the latter does feature some tasty playing by the Breckers.
It's really not even fair to compare the two, they're really apples and oranges.
But if you need to pick, I'd say go with the Hancock. But don't totally write off the Silver.
Check out samples at AMG or wherever and decide for yourself.