I have an Atma and have played the DS Mini several times. It really is apples and oranges. The Atma is an amazing multiple channel, multiple sound amp that with the right cabinet is a super-giggable/sessionable amp; especially with good glass.
The DS Mini has the most amazingly rich gain; comparable to the big brother DS, a real JTM45, a Soldano, etc. (I am not saying that it has the same amount of gain, just the richness/character of it is SO good). So if you are drawn to an XTC or BE100, then get the Atma. If you are drawn to an SLO or vintage Marshall, get the DS Mini.
I'm an XTC guy, so I got the Atma but honestly LOVE the DS Mini.