Hello Vdadleney,
even if they are only two sites, by using 802.1Q in Q you get the capability to implement VRF lite by adding an internal vlan when needed with no changes on provider side.
Another possible advantage is that if you need to extend specific vlans over the two sites you can do it (this is the original use for it)
The only price to pay is the increased overhead : provider devices need to be able to handle frames with an additional 4 bytes 802.1Q tag. (two tags)
Nowdays provider can give a link with very high mtu from the begining so the MTU test is recommended just to check that everything is fine.
multicast traffic can be used but IGMP snooping capabilities can be somewhat limited but this is a service provider issue that doesn't apply until there are only two sites.
From SP's point of view all traffic travels in the external vlan-id between the two locations.
QoS marking should also be supported with external 802.1Q CoS bits copied from inner 802.1Q. This is another aspect to be investigated with the provider. I would ask about this aspect to them.
Hope to help
Giuseppe