The undo/redo feature of Aperture can be used to quickly compare a set of different adjustments without creating new versions. I show it here using the crop tool, but it works with any of the adjustment tools.

Here is an image I selected and displayed in the viewer:

I press C to bring up the crop tool and then make a crop, but I don't press A to end the crop yet:

Now by making several alternate crops either with a new rectangle or by dragging the one that is already there:

I get Aperture to remember this series of adjustments in its undo buffer.
Finally I press A to accept the last crop and show it:

Now by pressing command Z to undo and shift command Z to redo I can go back and forth through the crops. This works best in full screen mode, and I can press F at any time to go into or out of full screen.

When I find the one I like, I just stop and go do something else. The alternatives are forgotten automatically. If I don't like any of then, I press C again and adjust or replace the currently displayed crop.