SPSS and SAS are two different statistical programs. Both have there advantages and disadvantages. If you are really into statistics and need to do mid to high level statistical operations SAS would be the program you would want to use. More recent versions of SAS are highly adaptable, giving the user the ability to customize SAS code (i.e. procedures in SAS language). In the past SPSS had a rocky start, but is a relatively good (not the program I prefer) statistical program which gives the user the ability to perform general statistics including descriptive, parametric and non-parametric stats. If I had a choice between the two I would choose SAS. A free alternative to SAS would be R...while its not one to one the same similar operations can be done in R that is done in SAS.
@ Fadia, Depends on your data and what you are tying to do (i.e. what hypothesis you are trying to test with your data). I am no SPSS wiz either, I have only used it a handful of times. I mostly use SAS, R, or JMP.
@ Soutrik, I have heard mixed reviews from people using Stata, R is pretty great if you know what you are doing, but I have never heard of using Python for stats. Granted you can write your own python code to do stats. Just never really heard/seem anyone use python exclusively for hardcore stats.
For using any statistical software it does not depend which one is better than the other, however it depends on your goal and your study and your factors and what correlations you would like to create so if any software answers all your research goal practically that means this software is better for you. even for example if someone would creates very simple statistics correlation Excell would be the best software for him and so on.