I have implemented about 4 NLB clusters for HT, CAS, or both now, and every time the rule of thumb has been, use what works.
From the MS Exchange page on IPv6 (also applies to IPv4) we know that:
- Unicast address A packet is delivered to one interface.
- Multicast address A packet is delivered to multiple interfaces.
More searching turned up this very helpful article describing the differences:
So, in one implementation, the customer had a Proventia M60 firewall, and unicast ended up working and performing more reliably. In another using an Juniper SSG firewall, unicast performed poorly and multicast ended up being the fix.
I searched the Exchange implementation guide and CHM thoroughly and didn't find any particular guidance, so I think at this point trial and error is the best bet. Learn what works in your config and go with it.