Its portability EASILY trumps the screen factor for me, and I honestly don't think the screen is particularly bad (especially once you get rid of a retina iPad that might be lying around collecting dust after you get the mini). Science papers are actually remarkably easy to read with zoom, and the best part is that many of the PDF readers recognize columns; double tapping on a column perfectly fits the width of the column to the display.
Nikhil72 makes an excelent point. The two-column layout makes it possible to read articles on the iPad Mini. However, you are spending 329 bucks anyway. I would chip in another 170 and get the best, most advanced ipad model out there. Then, I'll buy an iPad mini retina when its available.