Cisco ASA 5520 is the equivalent to the Juniper SRX240.
There are some other questions you should be asking yourself first before jumping into the SRX. Do you need SSL VPN's? If so do not get the SRX. I moved my SRX's to my internal firewalls and relocated the ASA's to the perimeter. If you are really needing the SRX for Anti-Spam, Mail filtering and Web content...(per my Juniper Rep) you should jump up to the SRX550. If you are limited to the 240 due to cost, make sure you get the SRX240H...the SRX240B does not support Content Filtering, Anti-spam nor Anti virus... It will support IPS if I rem right when they sent me the wrong one by accident. Also note that the SRX does not run Screen OS, it runs Junos latest release that I know of is 11.4R4.4
If you are planning on running redundancy, both Cisco and Juniper has their advantages and disadvantages. The SRX was a complete PITA, literally had to have Juniper set it up as you have to delete all the interfaces and then join the 2 nodes together then re-add the interfaces back in.
The ASA if you are going to run Active/Standby is pretty much straight forward, however if you plan on running Active/Active you will have to run in Multi-context mode which disables the ability for all VPN traffic.
I have also heard decent things about Palo Alto, maybe the PA-5020 but not that familiar with them at all.