There are two Stealthwatch products, on-premise and cloud. I have really only played with the cloud version.
When I compare what you get from the Meraki security dashboard versus Stealthwatch and consider the huge uplift in cost to get Stealthwatch - I don't see Steathwatch as a great investment.
You would really be wanting to monitor additional things beyond Meraki that don't provide the same level of visibility.
Also, you really want to be able to dedicate a full time equivilant person to use Stealthwatch to go through and analyse the data. If you don't have a reasonable amount of free time each day to look at Stealthwatch then you should probably be looking at some kind of managed service instead.
ps. I can't private message you at the moment. You might need to ask @CarolineS for help with that (assuming this wasn't deliberate).
pps. Scratch that, the private message option has now appeared today.