What type of plywood to use for a general purpose workbench?
Option 1 or 2. The plans themselves call for 1/2" plywood, and it's a general purpose workbench with good supports underneath, so 3/4" is overkill. Option 2 is a bit nicer of those 2, but again, it's a workbench, so you don't need super fancy. If you do plan on staining, pine does not generally take stain very well, so in that case you'd want option 2.

A fifth option, if you have any kind of hardwood dealer near you (search google for "hardwood" and ignore any flooring stores), is to get 1/2" Baltic birch plywood. This is the staple for shop furniture in many shops, given its nice look and low price tag; it also takes stain and other finishes nicely. It may cost more than the pine ply you linked (but may not!), but should cost less than the other 1/2" ply you linked.