I've heard the SME 20/12, 30/12 and used to own a 20/2. I also recently heard a AMG V12. So none of these were in the same systems so I can't draw any conclusions for you...
I will say each of them make beautiful music. At the top end, I am not sure you can really beat a 30/12...In the price range of the 20/12 and AMG V12, the Grand Prix Audio Monaco with a Triplanar arm (or other arm of your choice) is something to consider possibly...
One thing about SME is they've been around for years and a very stable company...
I am proponent of spending as much on the table/arm as you can...
What's the rest of your system?