There are a few physical differences, like bigger table keys on the Mark VII, and a standard high F# (optional on the Mark VI). It's a different body tube and neck, but that doesn't change how it feels per se.

As for sonic differences, that is a loaded question. They made the Mark VI for 20 years and there are definitely sonic differences over the years, and even from horn to horn within the same couple years. "It is worth it?" is a subjective question. To some people yes, to others no. And I know some people who do (despite what you always hear) prefer a Mark VII over a Mark VI. Really comes down to personal preference. My advice is play as many horns as you can get your hands on, and determine what you like (and what you don't). 

I agree with Jon. It truly is a personal choice. I have a Mark VI tenor that I have had since 1980. It was the only one available in my price range. I found it at the first place I looked, tried to find others, but couldn't .It was purchased in bad condition and needed refurbishing.

I could not have been able to afford one in playing condition. At that time they were selling in the 3500 range. I was a 17 year old kid and saved 625 bucks to buy the carcass. I was lucky to have a local guy that worked on all the working jazz players horns. When He was finished, he said it has a Sonny Rollins voice. That meant almost nothing to me then. Today, It is a true Gem and I agree it has a Sonny Rollins voice. He did the work for about 175 bucks. I still play on the tenor and would never replace it. I know it too well and it is now my voice.

I am sorry, I can get wordy.

My point is, If I had picked up a Martin or a Conn, I would probably still be playing on it also. I have a SA 80 series II alto that is a very nice horn. They are as good as the legend the Mark VI is. I myself find the larger keywork on a VII a bit over done, but it simply comes down to what you like or are familiar with.

Play as many different horns as possible and see what you like.