I had a Nova System, sold it, got the M9, sold it, got another Nova System.
The M9 was good, had great versatility and they can add/update models via software upgrade. I spent way to much time tweaking and comparing one effect vs. another, too many options - not a bad thing, but you need to take time to figure out which models, and dial them in which isn't always intuitive (use ears, not eyes). I think there are many usable tones in there, and the dirt models are fine (patience and tweaking), but in the end I found I liked the TC Electronic approach, and simplicity more. If the NS didn't exist, I'd probably have a M9. YMMV.
FWIW, I dig the Nova System enough that I purchased a G-System. I use the NS in front of my tube amps (usally a Fender), and my plan is to build a direct board using the G-System. I did consider getting an Axe-Fx Standard, but been there with an Ultra, tweaking and complexity for me. That said, I do plan to get another Axe-Fx eventually, once the gear fund heals.
YMMV. Get an M9/M13 from MF or someplace with a good return policy and give it go. IMO