It will be interesting to see, but if I had to place a wager I would go with GTA.
Both are games that sell by the boat load and have a large fanbase, but GTA will be fresher with the last having come out what, 5 years ago? people will be hungrier for a new one. Where as CoD will only be more of a side grade from the previous entries not really bringing much new to the table.
Lets put it this way, if you were a young kid who had $60 in his hand and could only get one when he loves both, which would you get the one which you last played years ago? will probably be very different to it, or the one you had a similar game of 12 months earlier?
The best way to find out if you can trust somebody is to trust them.