I have both an Elk alarm panel and an ISY. I converted my old alarm system last October and added the ISY late November.
While the Elk is capable of controlling lighting and can handle other automation tasks, the ISY is much easier to program and receive alerts from.
The ISY also works with Insteon. When using the ELK for lighting, I think you are more or less limited to using relays (there are probably exceptions to this). The ease of programming with the ISY, for me, makes it a worthy expense.
With the ELK, you are some what limited with the messages you can receive. The ELK will only send predefined messages.
I also found the mobile access with the ISY to be more wife friendly (we use MobiLinc).
One more thing, with the proper set up, you can access the programming menu of the ISY from any computer. I don't believe that would be the case with the ELK.
Having said that, there are some functions I automated with the ELK (using relays) for reliability. As an example, I have a zone on my oil fired burner for making hot water. That zone is tied in to my ELK and only runs when my security system is disarmed. I also send a signal to a virtual output so I can control this from the ISY with the ELK integration module.
I hope this helps.
Dennis