Menu Snapshot: This seems to be for local menus. A timed window screenshot is helpful for UI that have hover-state, such as dynamic JS/HTML menus or other popups.
Window Snapshot: This seems to be the same as single-clicking a window in 'Screen Snap' mode, except with an explicit prompt. It does not seem to offer a countdown.
I *am* looking for a "Window Snapshot" with a countdown mode, but since Screen Snap mirrors Window Snapshot, Timed Screen Snap should offer the same. Again, I distinctly remember this option being in Skitch a few years ago... it's possible I've dreamed it up, but