Each of them has advantages and disadvantages. If you know what you are doing, i.e. if you know diffraction, crystallography and the Rietveld method you can easily try at least two of them on the same dataset to check for solution stability.

I would therefore not talk about reliability as they are all tested fully and quite reliable, but about features and this in turn depends on your specimen and data.

Just as a side note, now with GSAS 2 you can write your own pieces of code too using python

All the three are powerful softwares. Full Prof and GSAS are free but TOPAS is a commercial one.

I did not use Full Prof much, but when comparing GSAS with TOPAS both have different methodology in dealing the problem. From my experience, the refinement in TOPAS is more stable than GSAS. That is what I like in TOPAS over GSAS.

Read through the elaborative documentation of these softwares.

I suggest u to get using TOPAS. If u like something in GSAS that is not available in TOPAS, you can write a user-defined macro of the same in TOPAS. quite handy...

Each of them has advantages and disadvantages. If you know what you are doing, i.e. if you know diffraction, crystallography and the Rietveld method you can easily try at least two of them on the same dataset to check for solution stability.

I would therefore not talk about reliability as they are all tested fully and quite reliable, but about features and this in turn depends on your specimen and data.

Just as a side note, now with GSAS 2 you can write your own pieces of code too using python

I have no direct experience with TOPAS so I can only say that everyone I know that bought it immediately got converted... maybe because you have to pay for it, therefore you won't go back to a free software after buying a not-cheap one.

Regarding GSAS and FULLPROF I have used both and I think FULLPROF is more complete and flexible, but flexibility may be a problem for unexperienced users. GSAS being a bit more limited than FULLPROF is much easier to use, the EXPGUI (Graphical User Interface) is very intuitive and for routine analysis I find it much faster to setup and operate than FULLPROF where a lot of text editing is needed to start a new analysis.

I fully agree with Matteo that the three pieces of software have been fully tested and are 100% reliable. IMHO it all reduces to the comfort (or discomfort) the interaction with the program causes you.

The need for detailed knowledge prior to operation may complicate startup if the program is not very user friedly (black box operation is a simple path for the beginner, dangerous though if not abandoned just in time :-)) but in the end there are many courses on the web and live where you can learn how to start and the tricks of the trade if you put just enough time and patience on it.

Dear Leopoldo Suescun, I´m an unexperienced user. You mentioned that there are many courses on the web. Can you send me the link, please? It will help me a lot. Thanks.

Try this for GSAS II

https://subversion.xray.aps.anl.gov/pyGSAS/trunk/help/Tutorials.html