From looking around, it seems JUNOS is newer than screenOS, correct?
Yes, there are two firewall software platforms ScreenOS and Junos. Juniper bought ScreenOS in the Netscreen aquisition and has been migrating these firewall features into the main Junos code since that time. Junos will eventually have all these features as a result.
However, the SRX240H appears to be EOL and EOS but I didn't see any such information for the SSG140, so I'm assuming it can still be supported by a contract?
The SRX240H PLATFORM is NOT EOL. Only some specific VERSIONS of early release hardware are listed on the EOL sheet. See these highlighted statements in the EOL notice.
Juniper will sometimes discontinue regional specific or hardware special version long before the primary project line. Attention to detail in the EOL notice is important, or your Juniper partner can help in identifying EOL hardware.
The Integrated Convergence Services (ICS) solution consisting of the Branch SRX Media Gateway products SRX210H-P-MGW, SRX220H-P-MGW SRX240H-P-MGW, SRX-MP-VA04, SRX-MP-VA04, SRX-MP-VA22 is being discontinued immediately, and removed as a supported product. These products were in early access, but not launched.
I was thinking of taking the SRX240H out of production and replacing with the SSG140 and obtaining a contract - but would this be a backwards step (assuming JUNOS is newer than screenOS)?
The SRX240 has generally higher specs than the SSG140 so I would stick with the SRX in this case unless there is some ScreenOS feature that you need not yet migrated.
Also, would there be any cluster capabilities between an SRX240H and an SSG140 or must the 2 products be the same model in order to form a cluster?
In both ScreenOS and Junos clustering you MUST have the IDENTICAL hardware for the clustered pair. And the clustering technology is different between ScreenOS and Junos so they would not be compatible in any case.