So, you already have the Maestro?? and what about your phono stage?? Will that drive the Shelter if you go that way?
I've owned both carts and feel they both offer a lot of performance at their prices. Your table/arm would be good for either.
Taking into account you matching to a good MM or MC phono section:
the Shelter is fast and clean , but with a nice warmth in the mids. A bit light in the bass compared to the better and more costly coils and also compared to the Maestro, which has a fuller or robust overall sound.
The Maestro sounds more MC than MM to my ears and has resolving power typically reserved for higher class MC carts.. Not as quiet a background as the Shelter.
So, it comes down to preference and what music you like as well.
I feel the Shelter shines with Jazz, acoustic and vocals. Piano is not it's forte nor is classical.
The Maestro is solid with classical and orchestra and does a good job with the rest..just not like a really good MC does there. But, I feel it closer to a very good MC than most MM's get. I own the Maestro, mainly because I have a vintage Thorens 126 and feel the MM is better matched, as well as my preference for Classical on LP.
If I had your table..I might consider the 501 and be very happy with that. I heard the 501 on my buddy's Nottingham Space Deck/Ace arm and was extremely impressed.
Ken