I'd go for the ecoboost personally - good torque and better mileage when not towing. That might not seem all that important now, but gas prices are pretty volatile.
Payload vs. towing capacity - keep in mind that in order to tow a heavier trailer, you also need more payload for the greater tongue weight ... The relative differences between these two options are probably not as big as they first look. (You might not be able to actually use the higher towing capacity of the ecoboost due to the need for more cargo capacity to support the tongue weight.)