I've had two SIM assessments with airlines one went well the other not so well. I'm lucky that I'm friends with a 737 TRE so could do SIM preparation for the second assessment. In both assessments they were mostly looking at rudder and stick skills i.e basic hand flying, pitch power couples (when accelerating / decelerating), raw data VOR tracking and ILS.
An awareness of threat and error management with CRM skills (NITS/DODAR) were needed but it was the flying skills they were mostly looking for. You can read up about NITS, DODAR, briefings and TEM fairly easily but flying an expensive SIM is not that easy.
I went to Virtual Aviation for a look around with my mate who was really impressed with them. He told me to go to VA but I was (wrongly) convinced it was the Multi Crew skills that were assessed in any future sim assessments more than the hand flying, so I went to a more reasonably priced one - saving £4000. I probably wouldn't have failed my first assessment if I had 40hrs on a 737 SIM and why I didn't take the advice of someone who runs airline SIM assessments I don't know.....
I understand you can now do a JOC/MCC on a A320 sim at VA - I personally would avoid this because if your Airline assessment is in a 737 you won't be prepared for the trimming etc.
I don't regret going to my MCC/JOC provider - the training itself was excellent but the basic SIM didn't really prepare me at all for an airline assessment, if I hadn't spent time in a fairly cheap 737 NG SIM with my mate I'd have failed both assessments. I used Airavana.co.uk at £55 per hour, its not a FFS but my mate said it was pretty realistic and good enough for what I needed.
Now theres the new APS MCC/JOC course I think its worth spending that bit more on a course to firstly get your foot in the door and then increase your chances of success at a future assessment. Whether that's with VA or another provider I can't say.