The reputation and back up offered by companies like SME are important factors when picking a deck, but long term satisfaction would be my biggest deciding factor. I went from 20/3 to 30/2 and don't recognise the flat, sterile sound suggested by some. Incredibly dynamic with a non existent noise floor. One of the easiest decks to set up but requires little attention afterwards, same for the V arm.
As with all these things, make sure you trust who you are dealing with. I'm guessing a dealer if 2 decks are on offer - one or two seem to specialise in them and can give you a good idea of differences and offer the back up.