They all "work". I'd get one with a helical focuser which makes life easy. Check also the size of the prism - that will determine the field of view. Then it's up to your budget!
Basically you need to place the guide camera chip at the same distance from the back of the scope as the main camera chip. Some people spend time carefully measuring that distance with calipers. I just have so many spacers around after using OAG's for 10 years that I trust I can find one if I need it. So I just eyeball it to get a quick starting point. NB -- It does get more complicated when using a focal reducer or flattener because now you not only need the two cameras to be par focal, but they also have to be an exact distance from the back of the reducer or flattener..
What I recommend is that people use the moon to get started - it's easy to find it in the guide camera (just slew to it and follow the light) and then you can see what you are doing. When the terminator is sharp you're close enough to go to stars instead. Otherwise, it's really just trial and error. Either you are too far in or too far out. What I do is start with the camera pushed all the way into the OAG. I then back it out in small increments using fairly high gain and use at least 5 second exposures. Once it focuses I lock it down. If it never focuses then I almost certainly know that I need more spacing for the MAIN camera. I usually prove this by moving the focuser in until the guide camera does come into focus.