I bought the Oxford to replace my twenty year old Webster's. I decided I needed a new dictionary when I wrote a story with the term 'Internet' in it. The editor questioned whether Internet should be capitalized - I looked it up and it wasn't in Webster, they didn't have the Internet back then, amazing!
So I bought the Oxford which has the reputation of being the most authoritative dictionary, but I soon found some differences.
In Webster, there is a pronunciation guide for every word, with the syllable stresses marked. This is invaluable if you are scanning poetry. Oxford only has pronunciation for uncommon and unusual words. So Oxford is not a good dictionary for English majors, or anyone studying metrical poetry.
I also found that the explanation of usage for further and farther was much clearer in Webster than in Oxford.
On the whole I found Webster to be better for common usage and has more variety of word forms. I haven't done a systematic comparison of the two dictionaries, but here some examples of words I have recently used in stories and needed to check:
coffee maker, is it one word or two? or hyphenated? - it's in Webster but not in Oxford.
skiff - 4 definitions in Webster, one in Oxford.
snowmobiler, that is someone who operates a snowmobile - in Webster, but not in Oxford.
damn well, is it one word or two? or hyphenated? - in Webster, but not in Oxford.
runabout, as a small boat - 2 definitions in Webster, none in Oxford, although there is one definition in my 'Oxford Dictionary for Writers and Editors,' 2000.
I did find some words in Oxford that were not in Webster:
small-town, is it one word, two words or hyphenated? - in Oxford, but not Webster
woodsmoke, one word or two? or hyphenated? - in Oxford, but not Webster
On the whole, I find Webster to be a much more versatile and useful dictionary than the Oxford. Between the two of them I have most bases covered.
The Oxford has more words in it, but also more words that I am unlikely to ever use, such as gymnure, gwyniad, gyttja, or gynmogene (an African hawk!) - and those examples are from one page of Oxford, so most of the Oxford is stuffed with rare and useless words.
As a writer I need a dictionary based on common usage, not on obscure definitions.
My next dictionary will be a Webster's.