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I would suggest you need more mount imaging capacity with either of those scopes and to 'future proof' you for imaging in years to come. Mount imaging capacity is usually regarded about 2/3s of the rated mount capacity. The Vixen sited weighs 13.2lbs for just the OTA, then add in the camera, guide equipment, finder, etc your at the limit of the mount; also at f9 your going to have to have longer subs to get equivalent results that a 'faster' scope could produce. The Orion weight is 8kg (tube and rings) and then add on the camera etc as above.
IMHO the Orion with its shorter focal length is a better nebula imaging tool.
You can image nebula and other DSOs with a newtonian, and will get true colour rendition and a fast f ratio without the expense of an APO type refractor. A carbon fibre 8" f5 newt would be a good match for the HEQ5, there are plenty of people using that sort fo setup. Good luck.
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