The SRX IPS is supposedly a one-for-one replacement for the standalone IDP, however it is missing some features to be sure (fail to wire bypass for example). The IPS engine is also not quite as proven in my opinion, although the Juniper IDP was never my favorite IDP either (mostly due to the requirement to run NSM). The SRX650-APPSEC-A-1 license you asked about includes the application security and the IPS licenses. My overall feeling is that if you need a combined firewall with IPS, the SRX is a decent choice (although it still wouldn't be my favorite for that role either), but as a standalone IPS box, it still isn't there.
Ron