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You're right about the prices. I only checked one dealer's website, but an XL is $999, a large is $699, and a small is $399. So to buy a large and a small would cost $100 more. That would be worth it in my opinion, but alas, it is only my opinion. Whatever they buy, they will be egging, and that is all that matters.
Unless you'll be catering or cooking for an army, a large should work fine for you. For the price of an XL, or close to it, you can get a large and a small, or a large and a medium. You will be able to cook 2 different things at 2 different temps. And sooner or later, no matter what you buy first,you'll probably end up with at least 2 eggs. Welcome to a cool cult.
The advantage is that it keeps your egg at the exact temp you want. The real beauty is that you will never be on here starting a thread saying how your overnight cook went south because the fire went out, and asking if we think your $40 hunk of meat is still safe.