firewall cluster

[sandra <sandra () fib upc edu>]

Hello,

We want to set up a cluster of two firewalls with heartbeat. It will be an active-passive 
cluster, so if main firewall fails, secondary firewall would become active.
We think that, although they are a cluster, they should have different Operating Systems 
(for example linux and BSD), so if a vulnerability has impact in our main firewall and 
drops it, the second firewall will start to serve without the same vulnerability affecting it.
Do you think is a good idea or is better to have two identical firewalls for compatibility 
issues?
Which combination of Operating Systems do you recommend?
Thanks,

Sandra