I have several Focusrite Scarlett devices.  One 18i20 is 2nd generation.  I did not find the new MixControl software nearly as intuitive (and therefore better) as I had expected, compared to the 1st generation.  As far as sound, the difference is minimal unless you plan on using extremely high sample rates.  I think the gain on a 2nd generation is perhaps easier to adjust in small increments, and the limiting seems better.
 
The same 1.8 software runs my 6i6, 8i8, and 18i20 1st gen units.  The Solo and anything starting with a 2 do not run the MixControl software, as mentioned.
 
Do NOT use any of the beta drivers.  I've been there, and had to uninstall them (no small task, even with company instructions).  1.8 seems fine.