@abij: No eReader ever used the
i.MX6S (for a very simple reason: it doesn't have an EPD), they all went with the
i.MX6SL and then the
i.MX6SLL, and both of them have a 256KB L2 cache.
TL;DR: Don't worry about that, newer is always going to be better.
The most relevant feature you'll get with the i.MX6SLL is the newer EPDC, with hardware dithering support

.