Hi Gav,
Sorry, don't have experience of both but I own the Star Adventurer Mini. I set myself a winter time task of capturing Barnard's Loop in Orion. I did this successfully using the SAM with an unmodified Canon 650d and a 50mm f/2.8 lens. I stacked about 60 x 2 minute exposures. The mount easily handled this exposure time at 50mm and I guess that one could extrapolate to say that 20s exposure with 300mm would be OK. Looking at YouTube videos I think that the star adventurer (not mini) is much more capable when it comes to longer focal lenghts.
I have seeen some nice images with SAM with 100mm lens (check out GWalles' gallery).
The advantages of the SAM are it's compactness and, in my opinion, the phone app. This allows you to set tihngs running and leave the system to get on with it. Surely the next version of the Star Adventurer will incorporate wifi connection and this app.
Probably not much help, but it helped pass some time for me..........
Ian