Welcome to OTN_
Read this article :
Edited by: Osama_mustafa on Dec 5, 2012 12:23 AM
This is a small question for which there are very long answers. Both are huge complex RDBMS systems and people have based their entire careers on one or the other, sometimes both. So I'll just give you one very logical answer I read once on asktom.com: SQL Server can only be run on the Windows platform. Not every company trusts Windows as a platform for mission critical systems. Oracle runs on mostly anything, making it a better option in many cases.
you probably refer to this entry at AskTom:
[url http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:1886476148373] AskTom: Comparison between Oracle and others
It is a long but interesting entry (lasting over eleven years, starting back with Oracle 8/8i).
Hopefully the comparison to other database-systems doesn't bother too much.