Dear @dover @drbond : In that SME review comparison exiast a very important difference that's that the 15A comes with the inferior 309 tonearm but my advise to the OP is to buy the 15 with the V tonearm.
The SME 20/3 has a much more substantial subchassis with a much lower resonant frequency and heavier platter than the 15. And of course the SME 20/3 is a significant improvement over the older 20/2.
It is a much better turntable regardless of arm, deeper bass and more gravitas in its overall presentation.