There is a list of apps: http://services.netscreen.com/documentation/applications/ and like the IPS signatures these are being updated all the time. On Juniper Firewalls, the AppID is used in AppSecure (in the Branch SRX from JunOS 11.2 onwards and HE SRX)
For comparison, you should do more than look at the list of apps though, see how the different vendors apply their application identification. A basic url or a port-based lookup will be less accurate than something that involves an application signature. Also look at whether you can customize existing applications, view application signature details, and create your own application signatures. Also look at the logging and reporting capability - the AppID logs via SYSLOG are detailed and well constructed.
A few unique things from your list that you might get with a Juniper FW and other Juniper Products are one Pulse Client for Dynamic VPN to SRX and MAG. Possibly the IPS/MAG integration if this is on the Roadmap (the capability exists in the Juniper IDP product range). Also if VGW is in your future for your Virtualized Environment then VGW/SRX integration might be of interest.
