Comparing the large to the XL, you are adding more space, however, you aren't limited on the Large in terms of what you can cook. A large can handle a packer brisket, prime rib roast, beer can chicken, etc. I've had 6-8 half pound burgers on my large at the same time. You can go vertical on the large to get even more cooking space. I'll be a large will meet your needs and save you money over the XL.
I wouldn't knock a medium either. I'd guess a medium would be easier to manage for low and slow cooks over a small.
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