They're both supremely comfortable, and I can effortlessly use both for hours.
I know I initially said I found the Series X controller more comfortable, but now that I've had hours upon hours with both, if I had to pick one, I'd probably go with the DualSense instead.
I think the Series X initially feels more ergonomic due to its smaller size and shape, but I found a lot of the controllers weight and pressure rests on the middle joints of my middle fingers as they hold the areas just below the triggers.
Conversely, whilst the DualSense is a touch too large, I think the weight distribution is superior. The weight is more evenly spread across my entire hands, with the main contact point being the upper palms, which simply makes more sense than having more weight/pressure rest on smaller contact points.
This is splitting hairs though. I feel like they're within a few percentage points of each other. I guess for me the perfect controller ergonomically would be something in-between the two, but they're both nearly there already.
I will however say that in terms of feel, fit, finish, uniformity of seams, build and overall noise dampening, the DualSense feels considerably more premium than the Series X controller, which is just far too noisy.