Unfortunately, not many will have direct experience in using AP mounts with and without encoders, so Rick's advice is gold IMO.
It makes sense that even a quality mirror system with a long focal length and long exposures will most likely be affected by some form of flexing. I would not even consider unguided imaging with long focal lengths, unless I was doing short exposure planetary imaging only.
Whether data quality is consistently better with encoder version when guiding - that would depend on a number of factors, such as seeing, imaging scale, exposure to wind etc. But perhaps I am stating the obvious.
Please keep us posted Chris
