What are some good references for preprocessing and analysis of the P300 response from EEG data in Python?
I don't exactly what it is you are after, but if you are looking for other analyzing techniques, you should try Independent Component Analysis (ICA). I have heard (but I do not have references) that ICA is better than PCA analyses. 

A very neat (and free) Matlab toolbox that has this function is FieldTrip (http://www.fieldtriptoolbox.org/). It is a nicely documented toolbox, including an easy to follow walk through and multiple tutorials, starting from reading in the EEG data, all the way to statistical analyses. FieldTrip is probably your best bet in finding the P300 differences, and still understanding what happens with the data (since you have to specify everything yourself).