as mentioned by @ashhong, a clean install would be ideal.
Reason: when you do an upgrade, it's adding on to your existing system, lots of the system files at present are old unused ones.
Then you will ask, what happens to my data? My practice before a clean installation is, do a full time machine backup then after a clean install, I will run migration assistant and only migrate 2 items from the list.
1 - Applications
2 - My own user account
the rest of the items, I will uncheck from the list.
The only stuffs that you will miss are system-wide settings such as network settings etc...
OS X: How to migrate data from another Mac using Mavericks - Apple Support