900srt vs 900nxs +SL4 for Professional Backline Company?

Hello,

I work for a professional  backline company.  We provide gear (drums, guitars, keyboards, DJ, etc) for professional national acts (and international) at our shows.  We'd worked with everyone from Lil' Jon to Busta Rhymes to Zed.    We are looking into purchasing a new four channel mixer for our dj set ups.   Now I'm not a dj myself, nor do I have any knowledge of actually using Serato, tracktor, etc. beyond setting up and plugging in the gear.  

So my question is which mixer would be more appropriate for professional users and specifically, which would satisfy more riders?  The choice is between the DJM900srt or DJM900nexus plus a rane SL4 box.    I'm open to all opinions, but especially if you are a professional (touring) dj or work at a production company. 

We currently have CDJ1000s, CDJ2000s, SL1200s, DJM800, and Rane TTM57SL's among some other more outdated gear. 

Bonus question, can anybody explain to me in laymen's terms what's up with Scratch Live vs Serato DJ?