Where did you find documentation stating tht Always-on requires AnyConnect Premium? The information I have indicates AnyConnect Essentials is sufficient. Quote:
"You can use a Cisco AnyConnect Secure Mobility license to provide support for always-on VPN in combination with either an AnyConnect Essentials or an AnyConnect Premium license"
source
Always-on is more secure in that you can restrict users from local network access if they are not connected on VPN. On-demand is a bit more flexible as the VPN only need be connected when access to resources at the VPN-secured sites is needed.
In general, AnyConnect Premium is required when you want to use Advanced Endpoint Assessment, Cisco Secure Desktop, or Browser-based (Clientless) SSL VPN. It also allows for the concept of shared licenses (a pool) across a cluster of ASAs.