The combination that I like (out of the variants I’ve tried) on the 301 has been an SME M2-12R/ Fidelity Research FR-7. For mono, I use an Ortofon RMG-309 (12” arm, despite the strange numbering) and an SPU G Mono. Another combination that I briefly tried and ‘worked’ has been an SME 312S with a Supex SD-909.
There are many other combinations to try yet, but I keep going back to the M2-12R/FR-7 set up because I love the rich purity of tone it produces and the way it excels with most types of music.
I’ve only had an SME V (with various cartridges) on the TD-124 so far, but I’ve had less time to play with that as my entire system has been boxed up for a year now. Anyway, I’ve recently made a 12” armboard for it, so much experimentation to be had.
I think, if I were really pushed to make a choice, I would buy one of Matt’s 301’s and put an M2-12R on it (very traditional looking arm, but with all of the benefits of modern bearings and precision engineering)
As I said, these are only my thoughts, plenty others will disagree