I really liked the shelter. Specific difference between it and the maestro? I remember thinking when I installed it, wow this is good. But in fairness, the maestro is a dam fine mm. I put the shelter on the summer of 2008, and was really impressed with it. I used it happily until early 2012.
The shelter worked well for me, and I haven't considered going back to mm. I find every time I go up the cartridge food chain so to speak, I hear finer details from the recordings. The shelter was more refined, and presented details in the recordings I didn't hear before. I thought it was quicker if you like, and more detailed than the maestro. It is as it should be. mcs should be quicker and more detailed. At the time I liked what I was hearing
I was then using a primare r20 phono stage into an antique sound labs 300b with 3a di cappos, (ambient tt). The system worked/works for me.
I am currently using a benz lp, which is just out of this world as far as im concerned. I have also changed the phono pre from primare to fosgate. Those two changes were huge, but ...see below, costly.
I think the shelter is a very good value cartridge. You get a lot for your dollar.(ie. it is not stretching the limits with respect to the law if diminishing returns). I was happy I bought the shelter.