Laptop, but in all honest I am still waiting eagerly for a good and fast enough e-ink second monitor, reading on a Kindle is such an improvement over a regular screen but the sluggishness is horrible ...
1. The screen size is too small for a readable, fit-to-screen experience
2. Scrolling is too sluggish to even try, you don't want to scroll horizontally and vertically on a PDF which is zoomed a few levels
Currently, laptop works much better I'd say.