new Ortofon TA 110 or Vintage SAEC 308L

The tonearm/cartridge match makes a very significant difference in how you perceive the sound of any "cartridge". Try listening to the same cartridge in two different tonearms, on the same TT, and you will see what I mean. Which is why I say the tonearm should be matched to the cartridge, not to the TT. And I am not merely referring to the tonearm/cartridge match as it determines the resonant frequency. There is more to it than that, but the mechanism must be very complex. I don’t claim to understand it completely. Not everyone would want to bother with this issue; most want to settle on one tonearm and choose cartridges that work with it in terms of resonant frequency. Nothing wrong with that approach.

I don't see how arms impart an audio characteristic.

Wait for it.....

I suspect they both match based on mass. The SAEC is a heavy arm and while I don't know the mass of the ta110, it was reviewed with a Cadenza Bronze which I assume has the same mass as the blue that I have. Perhaps I am being sucked into the audio black hole. Other than resonance and smoothly tracking, I don't see how arms impart an audio characteristic. My life would certainly be made easier by just leaving the 9" arm that is already mounted than going down the path of trying to mount the 10.5" which might require a new plinth.

The tonearm should be matched to the cartridge.  The TT does not care what tonearm is mounted on it; there is no effect on the performance of the TT.  So, I don't know what you mean when you say "better fit".  If the tonearm fits, use it, to paraphrase a well known homily.