Hey Geoff
sorry for the essay but when I started I always found it better to have more info than less, and this is a very common Q found everywhere so here it goes:
I've gone through quite a few scopes and have tried both. First and foremost that I wish I did properly is was having really good Polar Alignment. Regardless of OAG or Guidescope, a poor PA will give you really bad images and you will end up blaming the equipment (I should know; I did that!). Not all of us have the time or money or 'guts' to leave expensive equipment outside and in permanent observatories (I get scared just thinking about leaving my stuff in my backyard while imaging!). A really simple way of making sure you put your mount in the same spot everytime is to glue washers to the ground. Alistairsam showed me this and it works wonders. Even though my PA is still a bit off, every night I put my mount out and I can easily get 30sec unguided images, which is considered bare minimum to be able to do guided photography. It allows repeatability and once you get really good PA, you dont need to worry about it anymore.
From my experience I found that using a guidescope will only really work up to a Focal Length of 1000mm or so; from there the best bet is OAG. Up to a 1000mm, even a cheapie 400mm guidescope (Like the Orion 80ST) will do a good job with a cheapie QHY5, provided you have good PA. This is how I used to guide my 80ED, 120ED, and 8" Reflector (Infact I bought a $30 webcam, stuck it onto a 1.25 barrel, and that worked just fine to guide, and I never touched the QHY5 again. See the first three images below). I got round stars on most if not all my subs and I loved the simplicity of it. You have access to a much larger variety of stars since your field of view is significantly larger. Plus I'm not sure how much you do photography, but the f/ratio also plays a big role in how well a guidecamera will do. For example, with a guidescope at f/4 or f/5 a QHY5 will be just fine. At f/8, the image will be substantially darker and you will need a more sensitive camera. Alternatively you can increase the number of seconds a guidecamera captures before using that image to guide - but this relies on a good PA once again. If it's poor then your stars will be all smudged and your guidecamera will be unable to produce anything useful. I also never ever had a problem with flexture; maybe I just got lucky lol.
When I got my RC8, I tried using a guidescope with it - and it frustrated me to the point where I almost sold the scope off. I found it impossible to get nice round stars at 10/15min, and when your imaging at f/8 and on top the secondary blocks 50% of light, you can imagine 15min subs really are need on the faint stuff. Agreed that my PA wasn't very good then but I found the whole ordeal a nightmare with. Plus for a focal length of 1.6m, a little 80ST mounted on its back at 0.4m just couldnt cut it. And the idea of adding a 80ED or bigger meant my mount would be working ALOT harder and it wouldnt be very stable. That's when I got lucky enough to borrow a friend's lodestar & OAG unit. It was a bit of a pain to set up, getting all the right distances and adaptors all right. But once I got it working, with the same PA, I started getting perfect round-stars 20min subs (See the fourth B&W pic of NGC253). Even at 1.6m and f/8, I always found a star to guide on, using 1 sec or less intervals on the lodestar. It all just worked, and didnt get in my way of imaging. My webcam and the QHY5 really struggled to get anything however, and illustrates the point about having a sensitive guidecam. Alot of people shy away from buying these $600+ guidecams and then fail at OAG because they can never find stars so it is expensive.
Since then I've returned the lodestar to my friend and am completely convinced that OAG is the way to go. I think it is sage to say that the majority of users will have a similar opinion; low FL, use guidescope. High FL, use OAG. However there are always exceptions and I have seen amazing images taken with the RC8 using just their finderscope as the guidescope, and others using OAG at FLs of 600mm. Since you have a 8" SCT, I would say that OAG is the way to go. Though I recommend that you don't even use the 8" SCT. At those FLs, every single component has to work flawlessly and your PA spot on before any real images will come. Also the equipment will be quite dearer since they have to be more sensitive and of better quality to avoid hassles down the track. I take it from your post count that you are a beginner in astrophotography
and like me, the learning curve is quite large and insanely steeper when dealing with FLs that SCT bring. You'd be FAR better off learning with a smaller scope; everything is 10^23 times easier
and there are always great bargins to be found here. And if you don't like it or are struggling, this forum is amazingly helpful and the classifieds section really help us experiment with our equipment.
Anyways sorry for the essay! Hope all this helps. Just choose a path and stick it out. It may work, and if it doesn't, try Plan B
Clear skies!