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Off-axis guiders are great for telescopes prone to mirror movement such as SCT and some other Cassegrains as the guider is on the same optical axis as the camera. Should a mirror move the guider will pick it up and in some cases attempt to correct it (depending on how much movement is experienced).
Personally, I think if you're using a fixed mirror design or one that does not experience focusing/mirror flop such as the secondary focusing, you're better off with a dedicated guide scope. Make sure it is firmly fixed to the main telescope to minimise flexure.
Having a dedicated guide scope also allows you to guide on bright stars. With the off-axis guiders I've used, the light sent to the guider is dim making guide star selection difficult. The light is dim because a) it traverses a pick off prism to divert some of the primary optical path light to the guider b) the pick of prism is at the edge of the field of view. If your Newt has a well corrected flat field to the edge, then you might be ok. Most SCTs are very very disappoint off axis views. Stars are usually elongated.
The other items to be aware of are vignetting and parfocal. If the guider has a small aperture, you will be certain to experience some vignetting. This is not really bad as post processing with flats can sort most situations out, but try and avoid it if you can. In addition make sure the guider and main imaging camera are parfocal. If the main imaging camera reaches a different critical focus point (cfp) to the guider, you'll have have to match it (to make the guider reach cfp.
Finally, make sure you've got enough back focus to accomodate the off axis guider. Your focal length may change significantly.
Sorry, I can't provide any specific models or manufacture for the Newt OAG's. If you had an SCT, I would say you can't go past the Lumicon GEG. If you need to use one, the GEG is undoubtedly one of the best around.
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