Re: Which is better
There was a lot of variation in both the Mark VI and the Super 20 throughout their production runs. The most sought-after Mark VIes were produced between the mid 1950s and the mid 1960s. In my opinion, those horns are overpriced. There was also a wide variation in quality of Mark VIes produced within the same year, so the year of production is a far from certain indicator as to whether a Mark VI is a good one or not. Super 20s produced up to the mid-1960s were also fine horns, and they are more reasonably priced than Mark VIes. The fanciest ones were produced for about the first 10 years of the run and can be priced on the high side. So.... no clear answer on which is better, but you're more likely to find a reasonable deal on a Super 20.
The Conn Chu is kind of an odd beast to be comparing with the Mark VI and the Super 20, which were among the first truly modern horns. Its keywork is decidedly old-fashioned with poorer ergonomics and it takes more skill to play one in tune. Dynamic control is also more difficult. They're just not as refined. But if you like a big, ballsy sound and are willing to work for it they can be great horns. They're also significantly easier on the wallet.