Fabian -
I'd put my money on the two single interfaces over the VIFs, but I'm not a betting man. I prefer hard data.
To me the overwhelming issue here would be manageability and failover.
Do you realy want to configure alternate hosts to use different IPs ?
I'd go with active-active VIFs (ifgrps) any day over the single interface configuration.
I hope this response has been helpful to you.
At your service,
Eugene Kashpureff
NetAppU Instructor and Independent Consultant
(P.S. I appreciate points for helpful or correct answers.)