You can always download 13.04 iso image and try the live environment and see if you like it. There is no objective answer I suppose.
Of course you should certainly upgrade before the support ends and for 12.04 it means before April 2017. One reason to upgrade would be that as Ubuntu is coming to phones and tablets it must become more resource efficient. As people claim 13.04 delivers notable improvements in that sense. That means, it is likely to do the same tasks faster.
On the other hand, if your not an enthusiast who just enjoys new things and don't care if they don't work then it makes sense to wait before moving to new release. After first few months there is hope that most critical bugs have been revealed and fixed.
I am using 12.04 myself and really wondering whether it is worth the trouble of upgrading to 13.04. Even if you use the release upgrade feature it makes sense to make backups as things might go wrong (as you can read from various questions here) and the whole process takes time and effort. So maybe wait for 14.04 and upgrade next summer.
On the other hand if your current system has any chronic problems you might want to try if those have been fixed in the newer release.
EDIT: I upgraded to 13.04 and yes it is faster as everybody have been telling me.