With block signs you can make managerial moves while batting or pitching (such as calling for a 'hit & run' or 'squeeze' on offense and 'bunt press' or 'infield charge' on defense.) You make your selection before the action so you can focus on your batting/pitching.
Guide - Signs are chosen via menu based system. Clicking L2 (select if using PSP) will pause the game and you can take your time selecting the sign you want. See the block signs page for translations and how to use this option.
Sign Only - Signs are chosen by button commands, the menu will not be available. After you make your selection the sign will appear on screen as a confirmation. See the block signs page for a list of the signs and button commands to use.
Off - Signs are chosen "secretly" by button commands. The block menu is not available and there will be no confirmation on screen after you have made your selection.
In my opinion the only advantage of choosing 'Sign Only' or 'OFF' is that you won't have to know Japanese text since it's based on command. However, it's a bit more difficult and I personally prefer 'Guide' to pause the action, look at my translation guide and make sure I choose the right action.