I have the 22.5 WSM and the Onyx. I like both, but I would take the Onyx hands down over the WSM. Here are my reasons:
1. 7lbs. of charcoal will last you 15+ hours at 250 degrees. ( I have gotten 19 hours).
2. You can shut off the smoke and turn the Onyx into a oven.
3. Three sheves are accessible without moving racks.
4. The Onyx has a door versus a lid.
5. Wind is not going to impact the Onyx like it will the WSM.
Those are my thoughts (unpopular as most will find them).