Which tonearm should I keep?

One consideration is effective mas. Low mass arms run around 5-7 gms, intermediate mass arms such as the SME V, Linn Ittok and Ekos, Rega RB-300 and descendants, etc. run around 10-13 gms and are generally what most modern MCs are designed around.

The Fidelity Research FR-64S is probably the least versatile of the tonearms you have listed. IIRC when Hi Fi Choice tested it back in the 1980s it had an effective mass of around 35 gms, which makes it usable only with very low compliance cartridges unless you have a record flattener or a vacuum platter to flatten all your records. It's still relatively popular for such cartridges so you shouldn't have any problems selling it for a good price.

You don't indicate which other version of the FR-64 you have - the FR-64fx, which is made of aluminum rather than the stainless steel of the FR-64s has an effective mass of around 20 gms, which can be reduced using a lighter headshell (the standard headshell has a mass of around 14 gm). The Hadcock is a low mass arm IIRC and is supposed to be an excellent match for the Music Maker, which makes sense because the two are made by the same manufacturer. The Technics can be low to relatively high mass depending on which armtube you use, so it is probably the most versatile in terms of matching for different cartridges.

I stand corrected on the mass of the Hadcock, it is indeed a medium mass arm as Audiofeil says. I was thinking of the original Hadcocks, which were low mass arms. In terms of Hadcock and Music Maker, I only know what I read, which is that the Music Maker and Hadcock are designed to go together, and that the two have a connection via the owner of Music Maker (possibly British distributor of Hadcock?), although Bill (Audiofeil) may have more info on the exact connection than I do.