My email service of choice is gmail and also I have a primary in my business account as well. The reason being is because google has made it is easy for all of their users to integrate and link the individuals user accounts to all of what google has to offer. Think about it, if you have a gmail account you are linked automatically to google docs, adsense, adwords, facebook, just to name a few. Plus you have basically unlimited storage space and archive space in the email account alone.
It is mainly up to personal preference though.
I hope that this has helped you out some.