My instinctive response would be Gears 2, but I then had a wee look at the sales for the first games. Gears has sold near 5m copies WW, which Resistance has sold 2.3m WW.
The bulk of Gears' sales are in NA with 3.2m of the 5m there will the rest is in Europe/Others, with a paltry 66k in Japan.
Resistance has sold more in Europe/Others than NA and around 140k in Japan.
Also, we have to consider how different each sequel will be. Will Resistance 2 be bigger/better with noticeable differences to the first game? Well yes, it's look that way. What about Gears, well we don't know much about it to be honest.
IMHO, Resistance 2 will sell conderably more units in Europe/Others than what Gears 2 will sell. While Gears 2, will sell shit loads more than R2 in NA.
Overall, I do think Gears will over the first 2-3 months sell more than Resistance, but it will ultimately come down to what the game is like and how much of a leap it is from the originals.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)
