It depends on whether the attached network is sending you BPDUs on every VLAN, MST BPDUs on the native vlan, or just single BPDUs on the native VLAN.
In the last case you should be able to use either but with different vendors YMMV. If you use MSTP on the MAS it won't mix well with PVST networks and I've seen some strange behaviors. In the first case, use PVST.
If the network you are attaching to is running MSTP and you want the switch to participate fully in the network, you would have to set up MSTP on the MAS with the same MSTP configuration as the rest of the network and verify that the signatures on the configurations match (or equivalently that the link is marked as a "point-to-point" link not a "point-to-point boundary".) If you do not, the switch will operate as though the entire network is one bridge, which will work for basic stuff but nothing advanced.
The commands may differ between vendors, but the MSTP configuration block itself is standardized so this works between vendors.