As I said previously, I have not directly compared them. But it would not surprise me at all, if the Teres was not only in the league, but better. Beating more expensive turntables is what the Teres does best. I can't tell you for sure if it will beat the SME 20. But it does beat plenty of other tables up to $10k or more, so it wouldn't suprise me a bit. The price difference is why I recommend it so often, because the price is not an indicator of the performance of this table. I don't claim that it is the absolute best TT, but it is one hell of a good TT at a very low price.
As you know, I am analog-only and have a very revealing tube system, and I am very particular about sound quality. The Teres 245 is what I currently have, and I am very happy with what I am getting from it. The 255 is the top-of-the-line and will do even better. You could compare the 255 to anything made at the top levels, and the difference would be splitting hairs. When you factor in the price, there is no contest. Some comparisons I am aware of have included the Clearaudio Master Reference, Verdier La Platine, TNT MK5, top line Basis, and the Teres was either superior, or in the case of the Verdier, a tie. A Walker was found to have a slight edge in certain areas, in one test. No comparisons to Rockport yet. These tests were done by other people, not me. Including one professional reviewer for Listener Magazine. His conclusion was that,"makers of 5-figure American turntables should be very, very scared", when referring to the Teres. So what I would say is, that you can get real close to the top of analog for under $3k(w/o arm) with a Teres 255. How much more do you want to spend for that additional tenth of a percent that you might get with one of the few tables that can beat it?
I only point this out as a great value in analog. I don't intend to disparage any other analog rigs, because many are very good. It is the price/performance ratio of the Teres that makes it such a terrific value. I am sure the SME 20 would be a fine sounding table, and you would be happy with it. The same would be true of the Teres 255. As you point out, there is a large price difference.