We will be in Munich and Berlin and have time to see both concentration camps, but would like insight into what those who have been to both think.
The one thing you might think about is that if you go to Sachsenhausen, you might also want to visit Ravensbrueck, which was the womens camp. They are very close to each other.
http://www.stiftung-bg.de/gums/en/index.htm
http://www.ravensbrueck.de/mgr/neu/english/index.htm
I have not been to Dachau, so can't really say anything about it, though from looking at photos, it looks very similar to Sachsenhausen.
I'd just pick one. They tear your guts out. Two in the same day/week/etc would really be rough.
I have been to both.
I find Sachsenhausen to be the grittier of the two -- the tougher of the two to keep out of your head.
If you have trouble with images getting stuck in your mind, go to Dauchau, it is a little more 'museum like' and easier to keep "at arm's length".
If you want the most impactive experience you can get, between those choices go to Sachsenhausen.
I agree with Jo as well, if you go to Sachsenhausen, you might also want to visit the women's camp at Ravensbrueck. That will give you a more 'full look'.