There are a couple of things at play here....
1st - If this situation is like most retail oriented things I would bet that Samsung pays a certain amount of money to Verizon to advertise their phones. Since this is a new phone Samsung is throwing a few more dollars at it than in general.
2nd - If the above holds true then Motorola with a slew of backorders is not under as much pressure to advertise their product. Think about it. If you were a restaurant with the best chilidog in town that had people everyday lining up out the doors and down the street would you pay an arm and a leg to advertise more? You cant support the customer base you have now. Of course you advertise a little to keep the word out, but there is no need to run balls to the wall advertisements to get more customers that you cant help.
That being said, I think the X is relying more on word of mouth and viral marketing vs old fashioned advertise the hell out of it.