A shift kit modifies line pressures, and shift points. A good one can prolong trans life, as there is less slippage with higher pressures.
Typically, they still shift automatically when D is selected.

A manual valve body, is just that. You select the gear you want, and that's what you get, regardless of speed, or throttle position. .

Manual valve bodys are performance pieces too, so they will also have higher pressures, etc. for longer clutch life.

The sacrifice of both is firmer shifts, but for most of us with a muscle car, that is no big deal.