I assume by webmail you mean the web management interface of the two OS, ScreenOS vs Junos.
There is a world of difference between the web interface on ScreenOS and Junos. The Junos systems are mostly managed via the cli by engineers. The web interface is much better than it used to be but I still don't know any engineer that regularly uses the web over the cli for SRX/Junos administration.
Truely the web interface is one of the great advantages of ScreenOS over Junos.
But in you case the advantage of Junos over ScreenOS will be in routing engineering. When managing traffic on five different WAN interfaces you will have a wealth of more BGP options for traffic engineering than you do in ScreenOS.