I have recently been researching NoSql options. My scenario is as follows:
We collect and store data from custom hardware at remote locations around the world. We record
I am assuming if you have a lot of clients you will end up having a lot of tables. I would first remove this restriction and move to a single table model or having a table for clients and data with appropriate relations. This way you could keep mysql. Don't assume mysql is bad for everything.
In terms of NOSQL this depends on your data model and usage patterns but if you have lots of clients and you prefer this model then couchdb views could solve that problem as couchdb can support thousands of views. You can store all the data in one database in couchdb but have a view for each client. I have no idea how mongodb could solve this problem.