personally, i would consider the upgrade for two reasons, or i should say two new features, if needed.
one is the virtual firwalling, however, it doesn't support vpn yet.
and second is the ability to re-route the traffic back to the same interface. with 6.x, a packet from outside interface can't be routed by to the outside interface. which may be useful when configuring hub-spoke-vpn client vpn.
