I recorded the same scene in Far Cry 3 with both FRAPS and ShadowPlay. Both were set to record 1080p at 60 frames per second. The videos were combined in Sony Vegas and rerendered at 720p at 30 frames per second for upload to Vimeo.
To me, the quality looks very similar between the two, though the ShadowPlay recording has somewhat lower contrast and saturation. For these 30 second clips, the raw FRAPS file was 2.34 GB, and the raw ShadowPlay file was 201 MB.