In my opinion, unless you are buying a large number
of domains, it price should be a factor for consideration.
At most the price difference would be $1-2 or in extreme
cases $3.
I would recommend going for a registrar that is stable
and provides good support, etc. You should stay away
from certain registrars because there have been
reports of domains getting stolen/frozen etc.
Do your research before hand. Read reviews and user's
opinions and check in Google for terms like
"name of registar"+scam or sucks, etc and know the
real story.
Reference:
http://www.threadwatch.org/node/12636