I have already looked at it, all the links on google search is giving only pros and cons of each tool, based on that I think comparison is not clear. I am specifically looking for points which can show Splunk superior than Graylog2. Can you put some points from that perspective.
Answering your questions from 2 angles, commercial and technical.
Commercially, splunk is very establish in the security market. Today it has close to 3 thousands enterprise customer across different vertical using splunk for security and it is also listed in the Gartner, leader quadrant for SIEM. If you follow our splunk event, recently Mark Graff, CISO of Nesdaq OMX also showcase how splunk is being use for Threat detection. see here: http://www.youtube.com/watch?v=xV40H8zF1jQ
Technically, splunk is not just a log collector for different data sources. It is a data platform that is proven for easy onboarding of data with strong build in language for you to correlate against different data sources and perform proactive monitoring, predictive analysis and dashboard for different roles and department. See how splunk cops with SANS top 20 critical security control here: http://www.splunk.com/goto/Top20CSC
Hope this help.