I'd second avoiding the watchguard. Hideous Devices. the SRX range are pretty good, I've been assessing them for my company. They've made a good stab at making them work well using the GUI so that you can avoid the command line. Its OK (that was on JUNOS 9.4, I believe they might be on 10.0 now), however somethings, like configuring the ADSL pim, wasn't possible in the GUI. Do you need all the UTM features?
Dependant on requirements it might be possible to use a Cisco ISR with the firewall IOS, or if you need a good UTM device, then something like a Fortigate might be better, as the GUI is very well thought out.