It depends on what you want.
The CPU time will get you the exact time that your process is on the CPU. The second includes the time that your process is not into the CPU.
Imagine that there is a process that is using 90% of CPU, so your program will get a time execution of 5 seconds, but really it have been on CPU only 1 second.
The real time that you or the user will see is the total time. The other time can be used when trying to improve the performance of your code.