An OAG will certainly save you a bit of money when compared to a guidescope, but as has been mentioned, if you can afford a camera with an inbuilt self guiding chip, like a SBIG, then get one. The only downside of "through the optical axis" guiding is the small FoV in a large scope. That's where a fast refractor used as a guidescope is good. You'll get a decent FoV to play around with, so your choice of guide stars is greater.
In the end, your best bet is to go with what you can afford and what you feel you can handle
