Which cantilever to choose for re-tip

I would leave it up to Peter.  Both times I sent him carts he felt the best way to go was to just replace the stylus on the original cantilever.  And yes, he used what appears to be a blob of glue to attach it, but the carts sounded absolutely great when I got them back.  I can't tell you whether they sounded identical to the original, but I was very happy with the sound.  

The best philosophy is to stay with the original cantilever material because the cartridge was designed and built with that choice in mind. So your answer is “boron”. This has nothing to do with one being “better” than the other. 

By the way, I am only quoting the advice of Jonathan Carr, who designs Lyra cartridges for a living.  He also pointed out that there is a significant difference in mass between boron and sapphire, which would have led the original designer to choose different materials for suspension and damping of boron vs sapphire, etc.

Shelter makes great sounding cartridges; I just wish they had a rebuild, retip, or at least an exchange service - but I've never heard any examples of such a thing? 

What's a shame is that a brand new 501 Mk III costs only like $1000 or so from Japan, right? The $450 boron retip would get you back a Mk II at best, and that's close to the 50% cost of a newer Mk III that you'd expect to get back from a rebuild service (if Shelter had one price-structured like Ortofon and Koetsu). And of course the ruby cantilever will definitely change the sound, maybe not for the better. I don't like either of those options. I'd probably try to see if another retipper can keep boron for less money, or I'd get what I can for the 501 as-is by hocking it, and then buy a brand new Mk III from Japan.

Stick with the Boron.