They're both well put together, but the reMarkable is a clear winner. It's thinner, denser, prefers metals to plastic, and generally looks and feels like a more premium device. Due to the slim design, however, the reMarkable feels a bit more fragile. Fortunately, the reMarkable case is significantly nicer than the Supernote's. I also prefer the writing experience on the reMarkable, though they're unique enough I won't claim one is objectively better. (The reMarkable feels like writing with pencil and paper, the Supernote feels like writing with a ball point pen. Both are incredibly satisfying! We tried both, and I was planning to return the one and get us both the same model, but we ended up with different preferences.)
Thank you for taking time to write this! Based on your description, it sounds like reMarkable mostly matches my current setup. I'm a heavy user of (mechanical) pencil and paper.
Being an "excellent offline device" also matches one of my core requirements.