CFX or Fluent?
I want to use ANSYS for simulation of water turbine. Which one should I use for better result CFX or Fluent and why is it so?
That is a tricky question. I have (an older) license for CFX and think it is great for what it can do. 100M Element setups? Sure, why not. 1000 domains? Let's do it... Not 'simple' to use, but very powerful. That being said (and I'll just make the assumption that you yourself are not paying for either - take that as you may - so therefore cost is not a major driver), Ansys made it clear to me when I last spoke to them that they place more focus on Fluent than CFX and therefore you will see better capabilities (or easier workflows, etc) in Fluent. Keep in mind that CFX entered the Ansys environment a lot earlier than Fluent, so it is more entrenched in Workbench/etc.
Now... for turbine use. That is a quite specific field. Generally - and I may be wrong, again, not a user of the latest version - CFX is considered more accurate with turbo machinery and does have several options to fine tune things there and has the option to use a turbine-add on tool (haven't used that one though). Rotating frame of reference and moving mesh options are useful for these kinds of analysis, but a lot will depend on what it is you are looking at doing/investigating.
If I were in your position, the best option is the one that your local reseller is able to fully support you with (do they have an expert in either that can help through the finer details?). If you don't have one (!), it is whatever you can pick up quicker (likely fluent) and is more integrated in your other tools.
If I had to make a choice about this without any other criteria, my choice would probably be fluent, just based on where Ansys seems to be heading...