I have both (2018 11" and 2020 12.9") but still don't know which one I prefer. Kinda wish there was an in-between model, a 12" that could have the best of both worlds. Now that the new models might be coming out, I've been trying to figure out which one to get. I was going to replace one or both with 2021 11" and 12.9" models, but I'm not sure which way to go.
The 11" is light, portable, really easy to hold, and great for reading. But it doesn't display many websites in full width unless you hold it in landscape orientation, isn't great for side by side windows or productivity tasks in general.
The 12.9" feels big, immersive, has more screen real estate much better for any productivity tasks, especially if used on a desk. But it kinda needs to be used on a desk or stand of some sort, because it is unwieldly and hard to hold one-handed.