Flash modding iPods is a cool use case for larger-capacity SD cards. However, the limiting factors seem to be the database file for the songs on the device and the RAM available.
At a certain point you get diminishing returns on the card capacity as you couldn’t fill up the card with songs and have them all be indexed without the iPod crashing. In these situations, one can be fine staying at 128 or 256 GB.
I’ve had a Kindle Paperwhite in the past and now I have a Kobo Clara 2E (that’s the BW one).
I prefer the Clara because:
They both have water resistance and warm light. The Paperwhite’s screen is a bit bigger, but the difference is not monumental.
In the end, they are similar enough both in terms of specs and price that depending on your use case you will be fine with either one, but if anything I mentioned above is an important factor to you then I hope this was helpful :)