Aperture And Younity
For the impatient, my opinion is PhotoScope is the better Aperture tool right now.
Both tools require the previews to be generated. And both tools require your Mac to be powered on and connected to the internet.
PhotoScope has superior integration with Aperture and iPhoto - you see your libraries as you would from within Aperture/iPhoto. With Younity, you see your libraries as the Finder does (see screenshot at left).
Younity gains favor since it offers an "download" option - an offline mode, in essence. It's also WAN enabled. And supports more than just photos.
If Younity integrates with the OS X Media Browser and can scan and share 'Photos', that'd be great. Your entire set of images viewable when you're away (miles and miles away) from your studio. A great feature to have if you're with a client and the photo you wished you'd synced to your iDevice is quickly accessible with Younity.