I am sure you will be quite happy with 10.5 for the academic use. I use a 12.9, often in landscape, but then I am 40+ and eye sight get naturally a little poorer with age. I have no issues with portability as I always have a desk closeby to rest the iPad on. I annotate student reports in portait mode becuase reports typically have large fonts compared to those found in scientific articles. For labwork, 10.5 is likely a better solutions due to its smaller footprint.
It is also agreat device in teaching/supervisor situation as it works like a block of paper that you can sketch out ideas on and then mail to students.
If you use Mendeley, I can recommand Papership as it has better annotations tools compared to Mendeleys own iPad app. Do not expect to use reference manager software for writing scientific articles on the iPad.
Do you know that you can citate scientific articles on the iPad with either endnote or bookends?????