pdf organizer and reference manager

After being in the research field for a while, I found myself lost in a vast amount of pdf research papers.
I need a good pdf organizer.
especially when I start writing a paper, putting the reference  become a headache to me.
That's where Zotero and Mendeley come to help.
A comparison for these two:
https://www.youtube.com/watch?v=53QRA_SNSzE

http://www.zotero.org/
Zotero is open source, it  can organize other files like images and videos besides pdf.

http://www.mendeley.com
Mendeley is not open-source, it has a better support for pdf files

watch youtube videos on how to use it:
https://www.youtube.com/watch?v=xLtk6n8cFdk
https://www.youtube.com/watch?v=kSu4tO8Dwak
https://www.youtube.com/watch?v=VD1z0boSpQY
https://www.youtube.com/watch?v=XKAcQTea7E8
https://www.youtube.com/watch?v=zkrVbBSrK_w

I am currently using Mendeley.