Workstation is a user-mode program. It runs while you are logged on, and it won't run when you are logged off. So, it isn't well suited to long-running applications. It isn't appropriate (or even allowed by the licensing) to run Web or application servers in a Workstation session. By comparison, Server runs as a service independent of any particular user login. You connect a client GUI to a running session as required, and you can connect to sessions on remote Server machines as well.
If, like me, you run VMware on a laptop, you face the issue of what happens to your VMware sessions when the host's battery runs out of power, and the host wants to shut down. Neither Workstation nor Server does a perfect job of shutting down the guest sessions cleanly before the host shuts down, not in my experience, but Server definitely does a better job.
Workstation is a more appropriate choice if you are doing software testing primarily. In particular, you can create snapshots of your system as you are testing, and then revert to any of those snapshots as required. That can be a lot faster than having to do a from-scratch system setups for each part of your testing.
So, in summary, VMware Workstation is better for software testing, VMware Server is the choice for actually doing real work.