A large screen device would be the standard answer since it does not mess the formatting of, typically PDF, papers. However large screen devices are usually LCD and loses all of the readability advantages of the Kindle e-ink display.
For PDF papers to be read on Kindle, there are two great options. One is a program called K2pdfopt which reformats PDFs to be enormously more readable on Kindle. A wonderful program. Have a look at some of the examples in
this thread.
And the other is consider using koreader (requires jailbreak). It is a much more capable reader than the native Kindle and among other things will reflow PDF documents.
Ideally try K2pdfopt/koreader on Kindle & PDF papers on a larger tablet so that you can see for yourself which you prefer.