Hello,
the main difference I find between for a warm spare or stacking would be how you want them to reboot on a firmware update.
On a switch stack the switches will all reboot at once.
while with a warm spare I think you can do the firmware updates with a stagged update, so that during the update you can still have a switch up and working. Less down time.
I have some MS425 on my ESXI Clusters and I like having them not stacked so I can have no downtime during a switch firmware update. you just have to make sure you have links for both switches to work when the other is offline.