The best way to upgrade is to install a clean system. If you setup a separate "/home" partition, then just don't format it when you reinstall. If you didn't, make one when you reinstall. It will save you lots of headaches. Just backup everything that you don't want to lose (documents, pictures, etc.) and reinstall from a clean system. You might also make a list of all the programs you use.
As for which is better, it's all personal preference. I am using Mint 13 with backports enabled, mostly because I haven't had the time or inclination to install 14. One advantage to Mint 13 is that it's a LTS release, so it is supported for several years. If you like to tinker with your system, you won't keep it installed long enough for that to be a factor. Mint 13 is also supposed to be more stable since it is a LTS. You do get newer, more up to date packages with 14.
Running on a 4-slot toaster @ 60Hz