I've noticed a drop in quality in Moda Bella solids just since I started quilting a year ago. The same grey solid I bought about 6 months ago has a completely different look and feel than the one I bought 3 months ago. Definitely thinner.
One thing to keep in mind is that even within a designer's fabric collection, the fabric can feel different for the different prints. You will notice how drastic this is if you buy a precut like a jelly roll. One fabric in the collection will be really stretchy, thin, and have a lose weave. Another one will be thick and sturdy. So you cannot even go by brand, designer, or collection.
Michael Miller's solids are supposed to drape really beautifully. I only have a fat 8th pack so I cannot judge it, but it does feel silkier than Kona or Moda.