I run SW16 SP5. I don't often have very large assemblies but I *much* prefer it to earlier versions.
I rarely crash unless I'm doing something dodgy.
I don't think any version of SW utilises multiple cores.
I started out with 15 so I don't know about 14.
Why don't you upgrade one machine and test it out on your most demanding models?
edit: 2016 was unstable for me before SP5!
It might depend on which service pack of SW2016 you're entitled to. The initial release is when the icon colors were radically changed, which many users weren't fond of (and that's a significant understatement). We were given the option to go back to the traditional colors in a service pack (sp 3, maybe?). See here for a side-by side comparison (partial screenshot below).
Other than that, I'd advise looking at the "What's New" site to see what enhancements were introduced with SW2015 and SW2016 to see if any of them appeal to you.
I'm on 2016 SP4.0, the reason we're not on SP5.0 is because we didn't really need to move to 5.0.
We've been on 2016 since SP2.0 and it's a lot more stable then 2014 as far as it goes for me. SP2.0 had it's bug but it still had less then 2014 SP5.0 IMO. If you're able to upgrade to 2018 I'd suggest going to 2018, we are planing to do the switch soon. SP3.0 just came out this week.
I'm using SW2016 SP3 and have no issues with it, although I will say that I do not place the demand on it that many others might. Glenn is also correct in his statement that SP3 was the service pack where the icon colours were reverted back to traditional.
In assemblies created in SW2014, in equations with links between Global Variables: Part GV -> Top Level GV in SW 2016 will be deactivated and turned red.
To fix and reactivate such links all equations must be recreated again.
This is the only serious issue I was able to found.
Links between GV in a parts/assemblies will keep working fine.