I believe the stock rear end ratio for the Ecoboost is 3.31, not 3.10.
It's up to you man. You have 3 upgrade choices available. 3.55, 3.73, or even a 4.09. You never said what transmission you had which will make a difference because the ratios in the A/T are a bit different than the M/T. Wheel/tire size will also impact final drive ratios. Personally if you are not turning your car into a dedicated 1/4-miler I wouldn't go any lower than the 3.55's. The 3.55's will definitely get you up and going a bit quicker than the 3.31's.
A/T Gear ratios
1 - 4.17:1
2 - 2.34:1
3 - 1.52:1
4 - 1.14:1
5 - 0.87:1
6 - 0.69:1
M/T gear ratios
1 - 4.236:1
2 - 2.538:1
3 - 1.665:1
4 - 1.238:1
5 - 1.00:1
6 - 0.704:1
U.M.