Mike,
Both are current versions and at the moment there are no security/patch issues with either 2.6.5 or 2.7, so you have time to wait and see. If a security issue does show up, the latest version is always best supported.
Some folks have issues with their plugins not being compatible with 2.7, or they just dislike the UI changes. In my opinion, 2.7 is a huge improvement admin UI wise and once the theme is setup, threaded/paged comments are cool.
If you can setup a test blog then you can try before you buy. That way you’ll be aware of any issues before you take the plunge.
If you do take the plunge, make sure you backup your files and database and know how to restore it in case you get buyers remorse.