In the Aperture Preferences you can only set one external editor. The following Automator Service makes it possible to open a selected image version in another external editor. Just replace the call to Photoshop in the Apple Script by the editor you want. Also configure the export image format as you like it.

Download:

1. To edit the current Version: EditWithPhotoshopInAperture.workflow.zip
   

2. To edit the Master Image File: EditMasterInPhotoshopAperture.workflow.zip
   

The Workflow has the following actions:

Get Selected Images: select some images to be edited in the Browser,

Export Versions: Export the versions to a folder on the Desktop, to make them accessable to Photoshop

Run AppleScript: Pass the images to Photoshop, run a dialog to wait for the edit to finish; after editing in Photoshop, save the image and select if you want to reimport to Aperture. Be sure to save the edits before you confirm the dialogue - see the script below.

Import Photos: Reimport to a new project in Aperture

on run {input, parameters}

(* open in Photoshop*)

set numItems to length of input

tell application "Adobe Photoshop CS5"

activate

repeat with i from 1 to numItems

open item i of input

end repeat

end tell

display dialog "When finished editing, save the images in Aperture" & return & "and click 'Don't import' or 'Import' to reimport to Aperture" buttons {"Don't import", "Import"} default button 2 with icon 2

set the user_choice to the button returned of the result

if the user_choice is "Don't import" then

return {} --

else

return input

end if

return input

end run