It would be a very difficult comparison, as you suggest.
Not all Beaulieus are built like the 4008 series. I had a couple of 7008 versions, one of which was crystal controlled. Unreliable and expensive - they could not be trusted for professional work without a backup. I still have a 6008, but I wouldn't use it for a professional job.
The Beaulieus are not the only cameras with a 'proper aperture ring', and I'm not sure that a mirror reflex system/shutter is inherently better all-round than a beamsplitter.
My personal preference as the best all-round Super 8 camera ever made is the Nizo 6080. It is very quiet for a Super 8 camera, and can usually be found with a simple matte box/shade system.
The Leicina Special is worth considering - if it fits your purposes. I find mine to be very good for animation, time lapse/time exposure and stop motion. It takes Leica M lenses - which is not all that useful in practice. It is fairly noisy. Film speed is set manually (unusual for a Super 8 camera) and it has interchangeable viewing screens at the turn of a knob (unique among all reflex cameras, both movie and still?)
If you are going to transfer to video, and you wish to maintain sync for sound, it might be an idea to check what frame rates are available. Though most Super 8 cameras are not crystal controlled (ie accurate to between 3 ppm and 5 ppm - parts per million - in speed) they do have sufficiently good speed control for short takes in sync without using pulse tone for later synching.
Some cameras, like the Leicina Special and many of the Nizos, have 16⅔ and 25 fps as set speeds. That is 50/3 and 50/2 - ie each film frame can be transferred to exactly 3 or 2 PAL video fields with no skip and no fancy equipment.
One of the big questions should be whether or not you want the capability of shooting sound. If you do, then an old 16 mm camera like an Eclair NPR might be the thing. Extremely reliable, fairly quiet (quieter than the quietest Super 8 camera I've ever used - the aforementioned 6080 - but not studio quiet), takes C-mount and Cameflex-mount lenses, has a viewfinder that makes any Super 8 camera look like a toy, very fast magazine change with 400 ft loads, and heavy. For not much more money, but much better in a number of ways, there are the Aatons, beginning with the LTR varieties.
I think that the Eclairs and early Aatons are slightly better cameras than the early Arri SRs at the lower-price indy end of the sound camera market (ie cameras suitable for use with seperate sound) especially if you want to do a lot of hand-holding (ahhh, sweet), but Arris aren't so bad and they are much more plentiful in many places - Eclairs and Aatons are very rare in some parts of the world I've been to.
If you don't require a quiet camera or quick-change magazines then look at Bolex H16s. Real system cameras with a lot of advantages at remarkably low prices. They don't have pin registration, but they manage quite well without it.
Best,
Helen