https://wiki.ubuntu.com/LTS
- with non-LTS (Long Term Support) server versions they guaranty you free support (patches) for at least 18 months (1.5 year)
- with LTS server version you get five years free support (patches)
Since new versions of Ubuntu Servers are released proximately every 6 months you can always install new version and always have relatively secure server but I think that server that is patched for more then one year is more secure.