How well does a single A\C unit provide 2 zone cooling?
Using a single HVAC unit with zone dampers is pretty well-established and can save the trouble of having multiple separate units.
Some things to discuss with your salesman (and if s/he doesn't have detailed answers, ask for someone with detailed knowledge):


If your cooling needs will often only require one zone or the other (i.e., daytime downstairs, nighttime upstairs), will the system cool that single zone too rapidly to allow proper dehumidification?
will it have the capacity to cool both zones effectively when needed? (balancing this with the first point may require careful sizing of the system)
if the system fails, you have no cooling.   With two separate systems, you're likely to have some cooling in the house with one system running.
this might give you the option for a third zone, if your current ductwork allows (i.e., split one of the zones into two).   This could be a cost savings if you are currently cooling space that's not used full-time.


We've been in a house with three zones on one system for 13 years  and it's served us quite well.

I can't comment on that specific unit or the Nest.