I have the HQ Sweet 16 and used my DSM before that. I much prefer the HQ setup because like someone else said, you have a full range left to right and I very rarely have any issues with the throat space. Honestly I don't even notice it, but it would really bother me when it was to the left/right rather than at the top. And what that does is it gives me three sides "open".
And I tend to turn my quilts a lot! Depending on which way I want to go. I don't always work well upside down and that's one reason why I will never buy a long arm. The other is I don't want to constantly be having to roll up or down -- I like being about to squish the quilt wherever I want to go.
I've never even considered trying to change my orientation 90 degrees, but could on the HQ16 except that the table leg would be annoying!