It depends. If you're more familiar with one, go with that. If you're not familiar with any of them, then I would say that if your focus is on data and statistical analysis, go with R. If your focus is writing code and you also want to do stuff with your data, go with python.
Here's a nice infographic for the comparison: https://www.datacamp.com/community/tuto ... gs.Om1ZHyY
I suggest you choose R. It has every statistical test you've ever imagined implemented in one of its thousands packages. It's not presented in the same way you're used to with the other statistical packages you mention, but it can do anything they can and a lot more.
Some advice on how to start:
1. Install RStudio. It's not mandatory, but it makes R a lot nicer. You'll love how easy it is to make reports with RMarkdown.
2. Once you started R for the first time, install swirl. It includes lots of tutorials which you run with guidance inside R itself. http://swirlstats.com/