Hello,
My inital without analysis would be the Backup Server however you really need to do a perfmon for a minimum of 24hrs to really know which system will be the right choice.
Thoughts.....
The SQL component of VC having dedicated RAM could improve responce time if your ESX farm is large.
The Backup could buffer multiple streams of data much more effectively with the increase RAM.
http://blog.laspina.ca/
vExpert 2009