"That said, would you go with a 1.3 liter or a 0.9 liter size pot? I want the most versatility without going too big."
As previously mentioned, difficult to say without knowing your cooking style. If most of your cooking will be in a bag, then 900ml is more than sufficient. If you're going to eat family style on the trail and in pot, then I'd suggest the 1.3L.
Either of these sizes should give you plenty of options to do some luxurious cooking and/or baking on the trail.
"Also, it seems harder to find the Evernew pots in stock than Snow Peak, Vargo, or Toaks. Is there that much difference from one brand to the next?"
I have the Evernew 600ml paired with a Caldera Cone and I think it is excellent.
I also own a Toaks 700ml mug, 550ml mug, and 900ml pot. They are excellent too, much cheaper than Evernew, and easier to find.
I had a Snowpeak 700ml mug and thought that they overengineered the lid and handles and found that the mug was heavier than it needed to be. I can't speak for their pots.
I've never owned a Vargo product.
Now if you want to go UL and make every ounce count, the smallest/lightest I've gone for two people is by carrying a Toaks 550ml mug and sharing a meal with my kid/nephew that required no more than two cups of water and supplementing the meal with non cook items. You could buy two of those. When using Esbit or Coghlans fuel tablets, I only need 1 oz of fuel total to heat my morning drink and my evening meal. I could get by with .5oz of fuel if I really wanted to push it.
I carried this kit on my Wonderland trail hike this past summer. My complete kit including spoon, lighter, and all of my fuel was less than 8oz for six days on the trail.