Ah, well if you want to separate your traffic types, per path, that's more of a routing policy issue than a QoS issue. Often something like PBR (policy based routing) would be used.
That noted, I believe using QoS such that your traffic meets its service goals is the better solution.
For one thing, consider what happens if one path fails. Assuming you don't want to drop all your one class of traffic, in such a failure situation, traffic will be "mixed" on your remaining link.
For another reason, PBR can be a bit "messy" when dealing with lost paths.
Of course, it's also possible to use PBR and QoS together, but again, a good QoS approach often negates the need to policy route your traffic.
One of the best approaches is to used something like PfR (performance routing) which can dynamically load balance across multiple links and can insure something like VoIP is directed (again dynamically) across the path with the best performance for that traffic.
