SX2 and RX are nearly identical. The difference mainly to be found in the dedicated RB support for the RX, the SX2 still being able to support multiple software (although you have to set it to it’s “non-Serato” mode before it works correctly).
Personally I think the SZ is both too expensive and too big for what you get. At the price of the SZ you could start considering the MCX8000 which blows the SZ out of the water feature-wise, at least in my very humble opinion. Granted, I own one LOL.
I still have the SX (original, limited Gold edition = don’t ask it was a gear trade hehehehehe) that I have used for years and I honestly can’t say it feels toy-like, plastic or otherwise non-pro. Granted, the build quality is not of Denon fame, but then again nothing really is (the SZ isn’t either).
While the large size of the SZ jogs might be the thing you “need” most, for me jog-size has never been an issue. At the end of the day it’s still way smaller than vinyl and after 40 years on the decks I can adapt to any jog-wheel without any problems. I have, for a long time, used the MC6000 which is a really compact (19″) 4-channel controller aimed at mobile DJs that has relatively small jog-wheels (very high quality though). Never a problem.
We have written quite a few articles/posts on here on – what we believe is – a best-practice approach to buying a new controller.
Might want to look for a few of those, sure it will help decide. We do suggest not starting with the gear, but rather with your (intended) workflow, software of choice and budget and only then start looking for controllers that will fit those three things.