The issue is that editing images externally uses up huge amounts of disk space quickly. I use Aperture together with Photoshop a lot. I shoot more JPEG than RAW and when I edit a picture, the 5MB JPEG turns into a 30 or 40MB the TIFF or PSD version. I try to avoid it by doing as much as possible in Aperture, and I like the controls there, but stuff like Liquify has to be done in Photoshop.

I really can't justify 30-40 MB additionally per image. With the less important pictures, I want Apertur to send the file to the external editor in JPEG, or even better send it to the editor in a lossless format but store the result coming back from the external editor as JPEG. I almost never do multiple external edits, so this would be fine for me. Sometimes I even want to edit the original, i.e. I don't even want an automatic copy to be generated.

Is such a space-saving workflow possible in either program?