MARCOS were trained by the SEALs ,SEALs all train in counter terrorism as part of their training hence the MARCOS from the beginning picked it up and have evolved their tactics.
NSG may be dedicated to urban counter terrorism but like that so is the SWAT in US but tactics differ even though the NSG has SF guys on deputation but tactics differ from a military SF unit to a paramilitary unit like NSG,tactics like Method of Entry,Room clearance procedures,type of demolitions used.Since 26/11 Army SF has dedicated teams training for Counter Terrorism/CQB at the SFTS Nahan and they are on standby ,MARCOS practices a lot of CQB at Vizag.
NSG is a paramilitary force as it comes under the home ministry,it is a special response unit not a Special Forces unit.A unit capable of performing in all environments and capable of things like recon,intellegence collection,sabotage,demolitions is what is called an SF unit and NSG is only for CT you wont send NSG to blow enemy comms behind enemy lines hence it is not an SF unit.