Both units can play back and record S-VHS, not just the 40K. The 40K fan is much quiter, the loud noise (if you are unlucky enough to get one) comes from the tape transport while playing/ recording. Only the 40K outputs DTS (Stream option in the optical out audio menu). The 30K front display does have more info on it and is larger, but you can see the same info on the 40K's OSD- but it covers up a lot of the top part of the screen. The 40K chassis is also smaller and has both firewire ports in back, rather than 1F, 1R on the 30K. I haven't noticed any difference in quality of digital SD recording between the two, but the 40K does update the picture quicker between STOP-PLAY and FF/RW- PLAY switches. The 40K also has an extra D-VHS recording speed, LS5. In my limited use, I would not recommend using it unless you were more interested in preserving the audio quality than the video, or if you just needed to timeshift more than 15 straight hours for some odd reason. My first 40K & SR-VD400 had more issues than my 30K, it just depends on the luck of the draw, unfortunately.