What will the difference in speed be between an endurance geometry road bike and a race geometry road bike?
The main difference will be increased drag on the endurance model, as it is likely that you will have a higher handlebar position and this will increase your frontal area and in turn drag. However, if it is more comfortable in the more upright position, you may be faster as you are more efficient in putting down the power. 

If you are not racing regularly, my suggestion would be the endurance model, as you will be more comfortable, and if you want the low aggressive position that can be achieved by moving the stem/spacers around on the steerer tube.