Fog lamps do help.
As to hold on the car or sell it for a new car, you can the price you are getting for the car now, the fall in price after 2 years wont be too high.
   So if you feel you can afford the difference of the old and new car then you can surely go for selling it now.

Sell it now  and buy new sx4 zxi for great experience . yes, fog light increase visibility.

Honda City has fantastic resale value today and will have fantastic resale after a couple of years too. Since your car is not bugging you at all and after considering options like SX4, City ZX or Indigo, I think you should not sell your car now. SX4 is better on performance than your current car but comfort levels might not be as good. City ZX is as good or as bad your current car and Indigo doesn't seem to be a valid option here at all.

Fog lamps help you a lot in low-visibility night driving condition.

There are not many changes in the current Gxi model than yours.There are 2-3 minor changes namely fog lamps,electrically adjustable seats. Sx4 is no compitition to city in comfort level and technology.So,I think you should on your current car.Next year new city models are coming,so, you can try it then. Yes, fog lamps increase visibility at night.

Sell your car only if you want to upgrade, like Civic or Octavia, there is no point in selling your car for another City or the SX4.