I coded a bit in both and the difference is really not that big, as Marbelous said. Personally I would think Python 2.7 has more libraries compatible with it simply because it is older, for example nltk: http://www.nltk.org/
is only available for 2.7 as far as I know, not yet for 3.x versions. But it's really not that important.
EDIT: To see the differences, try out example code in your book in python 3.4. This will give you an impression of the differences between the two versions. If something doesn't work, look up how to make it work for python 3.4. Don't do it for every example though, only for simple ones.