This is one of the first things I changed when I upgraded.

In XP4, goto the Utilities menu and select Customize key short cuts...

In the list on the left side of the dialog, scroll down until you find "Toggle to Selector Tool." You will see that it is currently set to be Alt+S and that the entry above it uses the space key.

You can change the Alt-S to space, or add a short-cut so that either Alt-S or Space will toggle the selector tool.