It really depends on how is everything setup and what is your final goal. If you have an infrastructure already in place and you want to introduce the firewall without making a lot of changes, I think Transparent will work for you just fine.
Transparent FWSM will bridge 2 vlans and it is will "transparent to the users. They will be sending packets to the same MAC addresses they were sending before and the FWSM will "silently be in the middle".
In router mode the FWSM is a Layer3 hop in the middle and the hosts should be sending to it as a gateway or at least the FWSM will be in the path a layer 3 hop.
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...