I have istD and I had DS for 4 years.
istD has great layout for controls and ergonomics , way ahead of the DS.
Pictures with DS are a little bit sharper even in RAW, i am not shooting jpeg.
I read that some filter (forgot the name) that is installed in front of the sensor on istD is the reason for this.
Also DS is slow saving pictures to the card and istD is reeeeeeeeeeealy slooooooooow.
Take 4 frames in RAW in a burst mode and you can go make coffee, camera will be busy for a minute +
For some it will be a problem, I do not care much.
I sold istDS and I am still using my istD because of the ergonomics and the fact that i can use TTL flash with M42 lenses.
Oh and a few more things, AUTO ISO is really funky in ISTD, the screen is small 1.8 or 2', this features are better on DS.
But overall istD is a great little camera especially for travel and I still love it to the extent that my K10D is for sale now, but I will keep my istD.