The features of OSQA is listed here: http://www.osqa.net/learn-more/
And the features of Q2A are listed on Q2A site. You can compare yourself.
While choosing any script, my primary consideration is the platform, withour comparing apples with oranges. OSQA is written in Python and is powered by the Django application framework, while Q2A is written in PHP and runs on Apache web server.
So, I looked for PHP scripts which work on Apache, and made an extensive search (for PHP based open source, free, questions and answers scripts). I could find only three:
1. Qanda: http://www.qandasystem.com/
2. Qwench: http://github.com/anantgarg/Qwench
You can compare the features of these three scripts yourself by visiting the respective websites.
Of the three scripts I could find:
a) Q2A has more features than the rest two
b) Q2A is most actively developed among the three- compare the release dates and the date of last commit!
c) Q2A has the best support system, if you need support, post it here, and you get an answer, mostly within a day.
So, as on date, Q2A is THE best- among the specialized open source PHP based .uestions and answers script, which works on apache using MySQL! Only after this research, I chose Q2A for my site!