Both. After finishing a scene (and while writing it), I reread it on screen several times, revising and polishing. When I finish a chapter I print it out. There are some things you (that is I) notice only when I have a hard-copy, especially something like scene structure or examining if the different arguments/topics in a dialogue proceed logically or if I need to switch them around -- anything that goes beyond one page.
The polished chapter I then run through the spell-checker. Amazing how many "commited," "pursuade" and "imgaination" one can miss.
My sister read the first draft (back in 1996) and made a lot of useful comments. I read the first draft out aloud to my parents (who listened with friendly expressions on their faces but had no comments to make other than (my father): "I am surprised. It is better than I would have thought.")
Over the years, some of my friends read differents drafts (only one person read two different drafts, first in German, then in English -- while his native language was Dutch.)
Comments from these readers can help, but they have always been more of the kind what they like and dislike about the story, the characters, the setting. Polishing and copy-editing is my job; I would not trust a friend or family member with it (even if any of them were willing, which they most definitely are not.) My husband never read a word I wrote though he is more supportive of my writing than anyone else. He promised to read the novel when it is published.
P.S. Oh, and I've workshopped 5/6 of the first part of the trilogy at the OWW worshop. The reviews led to A LOT of revision and even some replotting. My opening chapters contain a lot more action and suspense now. With later chapters, the reviews became less and less helpful, because the reviewers had not read the previous chapters, and the synopsis I posted (if it got read at all) could not make up for it. Still, if my novel ever gets published, I will definitely name my OWW-reviewers in the acknowledgements. I would never have revised the beginning the way I did without their persistent complaints of "Boring! Too much dialogue! I want more action!" "What's happening here? I don't get it! Why is that relevant?" "Who is this man again? Should I know him?" "After 100 pages, I still don't know what your main character wants!"