I absolutely believe you could get 10 hours out of the MTP diffuser plate and charcoal ring. Using my MT with the diffuser plate has been so much fun and I love cooking on it. However, ever time I go back to cooking on the WSM, I think that is so much fun and so freaking easy to cook on. At this point I would not want to be without both, unless I upgraded to the E6.
I don't think you can't go wrong with one of the two larger WSM units or the new MTP. When grilling the MTP is going to give you a double kettle set up, where it and your performer can do all same things. This could be really cool if we ever get back to gatherings. When smoking the MTP will give you the whole grate to cook with, but the WSM was designed to smoke, so if you really want that dual set up one grill primary as you bbqer and the other as your smoker don't count out the WSM's.
When I got the diffuser plate system for my MT, some here where not sure how it would work for long periods as a low and slow machine. It has done great, and a few people said it surprised them, but now they believe it is going to work well for me. I am still more comfortable and confident with low and slow in the WSM, but the MT with the diffuser is getting ready damn close.
Enjoy this process, trust your gut, and get what you will be happy with! I am so glad I got the MT over the Premium, I never think twice about that. I debated between the WSM 14 and 18 and still pounder if I made the right choice.