I'd recommend sticking with 14.04 LTS.
The LTS stands for Long Term Support, which means that issues with software and security vulnerabilities will be fixed and maintained for 5 years, so until April 2019.
The next LTS release will be in April 2016, otherwise the standard support period is 9 months. So if you chose 15.04, you'd ideally want to move to 15.10 before the end of January 2016. More info about the release schedule for Ubuntu can be found here.
There may be new features in newer releases, and they are likely to support newer hardware, but an LTS is more likely to be stable for longer.
I think there's a 'move to new release' option in the software updater somewhere if you're set on the upgrade, but I don't tend to use the GUI for updating so can't point you there more accurately.