I’m only one data point, but I have had my SM3 and also used to have an SM7. Although, I initially thought I liked the SM7 as well as the 3, that feeling wore off and what you stated about your SM9 is how I felt about the SM7 which eventually resulted in my selling of that one. Of course, I only have had that one 7, so maybe it wasn’t in as good of shape as it appeared? I don’t know, but I haven’t had the desire to get another 7. 

As a side note, I also buy and sell typewriters and it’s worth noting it wasn’t easy to sell that SM7 even though it was in fantastic cosmetic condition and had the original brushes in their original green vinyl casing. All the SM3s I’ve had flew off the shelves and are always in excellent mechanical condition and have all felt very similar to me.

As far as I know the SM7 has basically the same design as the SM3 et cetera. In theory the typing action of the SM3 and SM7 should be comparable. However, we are talking about second hand, used machines with unique histories, so the typing action of these machines may vary a lot. In my experience no typewriter is the same, even the same models from the same year may have a very different typing action. 

I love my two SM7's.  They are perfect for me.  Touch control lever and even the spring tension for the carriage shifting can be dialed-in to ones likes.

I also have a SM3, SM4, and a SM9 from 1968. 

This one from 1961...
.

Pete, how is the feel of the SM7 relative to the SM3 and SM9? Is it closer to the SM3 or SM9 (or its own unique self)?

Thanks, Pete!