I want to serve my customers - Single edition or ESP?

With nuevoMailer SB/MB you can

Send campaigns on behalf of others by using a different Sender profile (sender details, sending method, landing pages, etc).
Share an encrypted link with your customers to access their campaign report.
Open different admin accounts with different roles. Like reports-only user, newsletter editor, list manager and more.
It is not multi-user. Single installation of files at one domain.
But you cannot give login access to the customer (in case you manage lists from many different customers).

With the ESP version

Single installation of files at one domain. Your customers (users) have their own accounts. Each account feels like and has practically all features of nuevoMailer SB.
Your customers design newsletters, manage lists and send campaigns. Only some higher level system tasks are reserved for you as "master administrator” (user accounts, email plans, smtp settings etc).
Of course you can still do all the work or divide the tasks with your customer.
Charging your customer is optional.

With both editions

The concept of "tracking domain” can be applied so that newsletter links point to the domain of your choice without actually having the mailer installed at that domain.

Another approach

Having different standalone installations at your customers' domains. You can do that either with the SB or the MB (1 year subscription license).
You can still do all the work or share some tasks with your customer. You can open different administrators accounts with different roles.
Consider this model when a customer has special requirements about data ownership and access.
It is cost-effective; you have 50% discount for each additional SB license that you order.
If you plan to have many installations or want the freedom to install anytime, anywhere consider the MB version (1 year subscription license).