Well they are 2 pretty different games. For example if you want to build large sprawling, realistic cites. Sc4.
If you want to build your own transit networks, like rail, highways, monorail, subway. Sc4.
If you want a game with over 10 years worth of user generated content, and literally thousands of Mods. Sc4
How ever, i have played sc2013 as well. Its not without its merits.
If your more about a functional small scale city simulation. Simcity 2013 will be enjoyable. Last i played, i was much eaiser to build a functional city for a beginner then sc4, and the stock buildings and stuff look pretty good.
Ultimately sc2013 got very boring for me very fast, you just keep bulding the same city over and over again. there is a point where your city Ends, and becomes pretty much unplayable.
If you have been a fan of the Simcity series, you may find sc2013 doesn't really fit the previous games, and hence it not being called Simcity 5.
I prefer sc4, but i am biased.
Edit: Sc4 is on sale on amazon