Hi Lee
Are you going to guide manually, (by eye), or via a ccd?
A Newtonian generally has a small amount of back focus, making it impossible to use an OAG unless you have a low profile focuser and low profile OAG.
An OAG generally produces distorted guide stars as they are often located at the edge of the FOV and are highly magnified by a Barlow.
If I did not have any problems with mirror movement, differential flexure, weight and balance, I would favour the separate guide scope. If the separate guide scope is also adjustable, it can make finding a suitable guide star that much easier.
With 35mm film I have used an OAG (Taurus Tracker II) and whilst it made OAG as easy as it can probably get, it was occasionally frustrating finding suitable guide stars.
I had none of that frustration when using a separate guide scope.
If you are going to use accessories such as reducer/correctors, then sometimes inserting these into the optical train with an OAG may require some tweaking to get things back into focus. A separate guide scope will not require any such tweaking.
One of the biggest challenges with a separate guide scope is ensuring that there is no differential flexure between imaging scope and guide scope. If there is, this will cause trailed stars.
Hope that helps a little.
Cheers
Dennis
PS - Here is a link to the newer Taurus Tracker III
http://www.taurus-tech.com/tracker.htm