Having a chamois (seat padding inside bike shorts) for spin class is a must, but classes are generally an hour or less, making them considerably shorter than half-day rides out on the road. This means you don't need all the bulk on your bum. The Spin Short ($55) by Moving Comfort softens my seat while biking inside.
Know for its sports bras, Moving Comfort is moving on to other types of apparel. These spin shorts are shorter than my usual bike shorts, but since spin class is usually hot, I appreciated the cooling effect. I particularly liked the zippered pocket in the back of my shorts. It's perfectly sized for my ID and gym card. I found these shorts really comfy and had no issue with seam chafing, and they wash up quite well; just make sure to hang them to dry.
To see how these shorts are sized
I think these shorts are correctly sized; I took a medium true to form. However, I did find the elastic used at the leg opening to keep the shorts in place a bit aggressive. It didn't irritate my skin, but it squeezed my legs a bit, creating a small sausage roll effect. This is enough to keep me from wearing them my spin classes, keeping my thick chamois bike shorts clean for my long weekend rides. You can get a pair a directly from Moving Comfort.