Either mode will work. The major advantages to deploying transparent mode are
1) firewalling non IP protocols
2) allowing routing protocol adjacency between a router on either side of the firewall althogh the FWSM does have some dynamic routing protocol support.
3) Easy insertion to existing network infrastructure
4) No need for NAT
If you want to deploy routed mode you will need to move the Layer 3 routed interface to the firewall and then create another vlan for the outside interface of the FWSM.
The major plus of routed mode if you are not running contexts is that you can have many DMZ's on one firewall. With transparent even with bridge groups you are limited as to how many vlans you can firewall.
It really depends on what future use you see for your FWSM, how many vlans you are looking to firewall and whether you are looking to deploy contexts within your FWSM.
It may sound from the above as though transparent has all the advantages but it isn't as simple as that eg. we run all our FWSM's in routed mode in our datacentres.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...