Hi,
I would suggest checking the 'new features' section of cucm 10.0 and 10.5
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/rel_notes/10_0_1/delta/CUCM_BK_C206A718_00_cucm-new-and-changed-1001/CUCM_BK_C206A718_00_cucm-new-and-changed-1001_chapter_010.html
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/rel_notes/10_5_1/CUCM_BK_CE15D2A0_00_cucm-release-notes-1051/CUCM_BK_CE15D2A0_00_cucm-release-notes-1051_chapter_01.html
Apart from offering new features, my recommendation is that we should always go for the latest version available as somewhere down the line you will have to upgrade due to a bug or a feature limitation.
Furthermore, you should check if there are any features in your current setup which you can not afford to lose out in the upgraded version.
HTH
Manish