Hello Haris,
FWSM:
can be used in multicontext, contexts can be routed or transparent.
FWSM failover option is to have one FWSM on chassis 1 and second FWSM on chassis 2.
the two chassis can be connected by a L2 trunk, you can have one vlan used for failover and one vlan used for stateful exactly as with an ASA pair.
>> I need only certain VLANs to be passed thru FWSM .
this is possible, only vlans specified in firewall vlan-group on C6500 supervisor are passed to the FWSM.
We use this setup in several server farms with good results.
FWSM can process 3 Gbps of traffic.
FWSMs cannot act as VPN terminator as an ASA
The ASA performance depends from the model. ASA 5580-40 outperforms FWSM but this is not true for other models
Hope to help
Giuseppe