The design features of the Mark VI changed a lot over its production run, so the differences between an early Mark VI and a late Mark VI are at least as great as the differences that marked the name changes.  There were changes to the bore, bow, and neck design for the sake of such things as improved intonation, tonal consistency, and dynamic range.  There were also minor mechanical changes.  Not all of those changes were for the better, depending on the tastes of the individual player.  

http://www.saxgourmet.com/mk-vi-production-variations/

The SBA - Mark VI change was largely ergonomics and adjustability of keywork.  Over the run of the Mark VI it moved from the darker, more spread sound of the SBA to a more modern sound emphasizing tonal focus and projection.  The Mark VI-Mark VII change was also keywork, but the design of the Mark VII continued to evolve.  The keywork of the Mark VII has generated complaints that it is "large" under the hands.  Some find the sound of the Mark VII shrill and thin, but some say it sounds just like a Mark VI - which leads to the question of which Mark VI and which Mark VII.  

It is important to keep in mind that every series or generation of Mark VI's had a slightly different sonic character. This is a generality and not a rule.  There can also be a lot of variation from horn to horn.

The Joe Henderson album State of The Tenor showcases him on a Mark VII. He seems to have a deep and slightly heavier sound on that album. Just food for thought.

Based on the posts and who has posted, it is very evident you are getting excellent feedback to your question.

The fact of the matter is we saxophone geeks look way too much into things. If we all spent as much time practicing as we did analizing, we could push our art much further.  The best advice I can give, someone who has dealt with many horns of all types, is to play a horn prior to purchase. If you do not find anything that truly is an issue, the price is right, and you like it, you got a winner. Every player sounds different and has their own voice. Mouthpiece combination along with working out with long tones is how you find the sound you are looking for. That sound in your head.  Even Sonny Rollins practices 3 hours a day and is what 85 now?

Play On!!