Fair comparisons of different versions...

rutt

...of the same shot. It seems easiest if you see them in exactly the same place and toggle back and forth. There are a few ways to do in Photoshop, but both versions have to be exactly the same size (pixels in each dimension), bit depth, and color space. If so:

1. Make version 1 active,
2. Select all (command-A), Copy (command-C), Deselect (command-D)
3. Make version 2 active
4. Paste (command-V)

Now you have several different ways to toggle back and forth. You can use the layer palette and toggle the eyeball of the top layer to show either version. Or you can use the undo key (command-Z) to toggle by undoing/redoing the paste command.

Important. Be sure you have both versions saved out on disk before you get carried away with this.