Cartridge or cable upgrade

I'm not sure what you're considering would be a really audible improvement over what you have. Personally, I would try a Herbies mat before doing anything. It made a very audible improvement on my old Music Hall (Project) tt, but it seriously degraded my Linn, IMO. So you just have to try one and see. Herbies is always money back, so there's minimal risk. BTW, the mat improved the Music Hall much more than you would guess. More solidity, very improved bass response. Just all-around better.

About the Herbies Mat - you asked what thickness to get. If your tt has a thin felt mat, get the thinnest one. Herbie will help you with this. I wouldn't normally recommend a mat as a first upgrade, but on my MM5, it was ridiculous how much it helped. I'm not even a mat person. I don't use one at all on my Basis.