The French horns, the Selmer in particular, and their Chinese clones are often referred to as " Uber-Baris" as their tone, due to their less voluminous bore, lacks some of the sonic presence of the larger bore horns. While the big-bore/small-bore argument proponents on each side when it comes to altos and tenors, the arguments is not so balanced when it comes to the bass. It seems to be a pretty widely held belief that the extra sonority of the bigger bore is more important when it comes to belting out low notes on bass.
That said, there are minimal differences between the MVI bass and the current SA80II model. The MVI will only be keyed to Eb and some models lack a Bis key. However, the bore dimensions and tone hole size/placement are extremely similar if not identical. This leads to far better intonation, especially of the bell tones. While the middle register (particular D) can still be somewhat stuffy, it is much less so than on the classic American horns. Even though the keywork may be somewhat limited, it will be much more ergonomic and comfortable than any pre-war bass.
Value wise, the French instruments, in general, tend to be worth somewhat less than their American counterparts. However, the Selmers are the exception- probably as much on the Selmer name as on their rarity. While a Conn or Buescher in excellent condition will have a retail value around $8,000USD (+~10% for silver plate), a Mark VI bass in any decent playable condition seems to list for $10,000+USD. I'd say at least at least $12,000USD for a pristine example. Silver plate is so rare on these that it might add 20%. Keep in mind that these are optimal retail prices. For a private part sale, you should expect 70-85% of these values, depending on how long you want to keep it. Also, the bass market the way it is, all of this is more academic and the horn is only worth what someone is willing to pay!