Which of the chat tool is good for a SaaS?
I would go for Drift. They provide plans for all stages of the startup.
The best thing is, they have a free forever version which is perfect for individuals/start-ups who want to have a chat functionality on their website but cannot spend as low as $20 per month for it.
Also, if you work at a Marketing Agency and you have a lot of clients who want to implement this functionality, Drift allows you to manage them all in one account and have different billing plans for different clients according to their needs.
Additionally, they also provide support for chat bot which is a cherry on the cake. The people at Drift have made the customization easy so it can be implemented quickly.
Go with Chatterbox; it is part of an email marketing solution which is free up to 2000 contacts. Best thing is that it is integrated with slack, you will not need to login to any other application to reply to your users chat. More over, any one in your slack team can respond to that chat (at any time, from any device). The contacts of Chatterbox can later be nurtured to leads and then turned to customers via their drip.