Sean,
I will not claim to be the most knowledgeable person on these forums but in my opinion I would go with an ASA 5520. If redundancy is major concern of yours and your companys then I would suggest getting a pair of ASA 5520s and placing them in an Active/Standby configuration. This will allow you to be completely redundant in case an interface failure or if the ASA goes down completely. As far as monitoring tools the ASA does have some built in de-bugging and logging features that may be able to help, but also snmp is a pretty standard technology for real time monitoring of your firewall. Just because it took me a while to set it up I included the snmp configuration at the bottom for you to check out. This is the configuration on our ASA 5520 currently and we use Paessler SNMP Helper (http://www.paessler.com).
snmp-server host Inside 192.168.x.x community XXXXX
^--- sets up only host 192.168.x.x can obtain snmp data
snmp-server community xxxxx
^--- open to everyone if you want
snmp-server location Home-Office
no snmp-server contact
snmp-server enable traps snmp authentication linkup linkdown coldstart
snmp-server enable traps syslog
snmp-server enable traps ipsec start stop
snmp-server enable traps entity config-change fru-insert fru-remove
snmp-server enable traps remote-access session-threshold-exceeded
Again I am not positive this will help you make your decision but it should point you in the right direction. The reason I didnt suggest a PIX is because it is an older technology (relatively compared to the ASA) so its better to buy the most recent equipment. Of course your budget for this project will be a deciding factor =)
Good luck with your research (please rate this post if it helped you out at all)
Chris
