The units are both diffrent in terms of usage.
The 1240u Photo come with a adapter to scan photo negitives and is ideal for scanning in these type of images.
The 1640su is the next model up in the scanner line but without the adapter. it is avalible with the photo negive adapter (1640su Photo) but if you just want to scan standard prints of printed photo's then the 1640su has the highes quality. also if you have a scsi adapter then the 1640su is also faster.
Only the insane prosper.
Only those who prosper can judge what is sane.
Oh, so <i>that</i> is what "Photo" really means! Thanks a lot.