As someone who has used both, it really depends what you're doing with it. Buy the 13" if you intend to replace your laptop with it, or use it mainly on a desk. Buy the 11" if you plan on supplementing your laptop with it, or if you're using it more on the go, notably more in the tablet form factor.
In 2018, I bought the 11." As it was a laptop replacement, I wasn't really happy with the size, but kept it until 2020 and got the 12.9." Which, I think I am more happy with. Eventually bought the M1 12.9" (which I eventually traded for a MacBook, anyway).
I'm more open to trying the 11" again as a laptop replacement since external display support is available. Things don't have to feel so cramped at my desk. I'd have to try my mother's 2018 11" iPP side by side with my spouse's 2020 12.9" to really get the feel of which I like better. Which were both ironically my old iPads, would be weird to use them together.