Metal tapes generally have the highest frequency response, with Type II high bias a close second.
A really good cassette deck will have a high frequency response in the range of 16-18 kHz using Type II tape. Metal tape can be as high as 20kHz, and normal bias form 12 - 16, depending on the tape deck and noise reduction used or not used.
Decks from Nakamichi, Tascam, Denon, etc, tend to sound more open with better high frequency response.
Higher is not necessarily better because there are other factors that come into play. Personally I would skip the metal tape route and stick with TDK SA-X or Maxell XL-IIS (if you can find it.)
-Tim