The biggest factor to influence your choice as has been said is which language you're most comfortable with. If you're relatively new to both languages I would highly recommend python, the C++ code that is used when using ROS is quite advanced using many complex features of the language.
Another thing that should be considered is that the ROS python API is slightly more restrictive than the C++ API. If you're getting started with ROS and working on hobby projects then this limitation of python will probably not effect you, but it's worth bearing in mind if you move onto more advanced projects.
Regarding your question about speed. The speed of python will only effect you if you're writing your own complex algorithms in python. Using the python API all the ROS functions will still be executed using fast compiled C++ versions, only your own code will be executed using the python interpreter.
Hope this helps you make and informed choice.
Whichever you're more comfortable with
ROS wiki provides pretty educational tutorials for both c++ and python. It is just the matter of which language you've got experience or comfortable with, either with syntax or compilation!