I'd go for the upgrade. You'll receive a pop-up in about two weeks with the very question and all you have to do is click OK and enter your password. Upgrading gives you an identical system as you get with a full install, but the advantage of upgrading is that you don't loose installed programs and your configuration files.
Only if you've been making a mess of your install and want to get rid of things that you broke experimenting, then a full install is advisable.
Forget what you read about Windows upgrades and a full install being better. Upgrading Linux works in a different way.
Although all your files should be fine during an upgrade, always make sure you have a backup of you personal files before upgrading/install on a different disk/dvd/usb stick/...