you may want to check this post before moving on:
Creating a new database vs a new collection vs a new cluster - Ops and Admin / Installation & Upgrades - MongoDB Developer Community Forums
collection per client will give flexibility for indexes plus queries will be faster as you will be searching only on that client’s collection. Yet there is a limit (about 10000) on the total collections you can have in one database. so if the number of clients you will have may go above that, consider the next layer.
database per client will give more power if you want to allow clients to have still-relational data as the tables in an xyzSQL server corresponds to the collections in MongoDB.
database is the highest level in a mongod instance, and instead of scaling database your collection resides, you can create new clusters if the number of clients starts increasing. you can even group client types into clusters.
things to consider can really be overwhelming. but again, take your time. as long as you keep a backup, feel free to experiment with ideas.