I think you would be better off with a higher res/bigger screen if you only want to play casual games i.e. non competitive games such as CSGO, COD, Battlefield, etc. Competitive games would benefit from the higher refresh rate and lower response time (see something faster, kill something faster). Casual games will look great with the higher res/bigger screen even if it doesn't have G-SYNC but still will look good with the other monitor if you really want G-SYNC. Also consider if your graphics card could handle higher res.