Unified backbone, or separate?

Separate backbone
No SLA comproise to IPv4 services - operators are happy
More equipments to babysit
Separate bills for leased lines - not pretty
Can use IPv6 routers w/ beta firmware, if your IPv6 SLA allows

Unified backbone
Chances for SLA compromise to IPv4 services
If IPv4/IPv6 dual stack router is not stable enough
Less equipments to babysit
Single bill for leased lines

(tunnel is a complicated beast - be warned)