- don't know if there is any difference in accuracy for GLONASS and Galileo - who do you trust more Russian or German? ;-) - Finally you can try out if it makes any difference to you
AFAIK Gailileo is not fully operational yet (http://www.esa.int/Our_Activities/Navigation/Galileo/What_is_Galileo). GLONASS is a good addition for high northern latitudes and other places where signals might be attenuated. My suggestion is to see what works best. Adding additional satellites comes at the expense of battery life so if plain old GPS works fine, stick with it.