I always read online,and thats only because i cant get t harder them in my country and its even harder to get them in english I'm pretty sure thats the case with everyone,scrolling down a page annoys the crap out of me.
Physical copies are better but of course are pretty costly. I don't have a huge collection but I do have a couple full series. I would prefer 2 or 3 full series over a scatterload of manga with skipped volumes.
The first manga I ever read, me and my brother both chipped in to buy the whole printed series (Rurouni Kenshin). There's always something hugely satisfying about being able to sit down and flip through the pages.
That said, because I currently live in an a very small apartment I simply do not have room to handle printed copies of everything I read (everything from manga to novels). Digital is simply way more convenient.
My dream is of course to one day afford a house big enough where I can have a study room dedicated to bookshelves of everything I've ever read. By that time however I think I'll be so used to digital media that they'll just be professional dust collectors.