I am looking to get a new road bike and am wondering if there will be a difference in speed between an endurance geometry bike like the Fuji Sportif or Specialized Diverge, and a race geometry bike like the Fuji Roubaix or Specialized Allez. I am looking to go as fast as possible but I also plan on going on 50+ mile rides so comfort is also a factor for me. I also do not plan on doing many hardcore road races. Other info: I am 140 lbs.
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.
If cranks, saddle and bars are in the exact same relative positions on both bikes and all other factors remain the same - then there will be in theory - no discernible difference at all. Bicycles labelled as endurance road bikes are not necessarily any slower than a full-on race bike. And the lines become blurred when you consider endurance race bikes.
In general (very general), an endurance bike will have tweaks to the geometry & design features leaning towards comfort.
- taller head tube
- shorter top tube
Both of these can provide a more upright ride. But fit a longer stem and remove the spacers and you're in the realms of an outright race position again. More importantly, an endurance bike may have more compliant ride characteristics built into the frame. Such as thinner seat stays which flex and absorb the road buzz. Take a look at the Scott Solace or Specialized Roubaix for examples. Raced on the cobbles but endurance road bikes nonetheless. This is just two examples of many endurance race bikes.
I should also add an endurance race bike will maintain the close clearance geometry on rear and steeper head-tube angle. Which will keep the bike responsive.
Moving away from endurance race - then the bikes may take on more of a touring geometry also - with longer chain stays, slightly slacker head-tube also. They may also allow for bigger tyre clearance, disc brakes and mudguards.
If you're looking to go as fast as possible with consideration for comfort - than it is an endurance race bike you should be looking at.
Apart from the different geometries, tires, weight etc, etc there is a main difference between endurance & a pure sport/race bike. It is the stiffness regards bike flex under pedal load. A sport/race bike will flex less because it is designed to flex less in the carbon layup plus usually it is shorter with less tire clearance so again less flex. Will not have a flex seatpost etc as any frame movement between riders backside & pedals translates to torque loss. Basically the stiffer the bike the more torque can be transferred to the back wheel & again the stiffer wheel can then transmit that torque to the tire. Same again for big tires, they flex rotationally under load which is torque loss which is why on a moderately smooth surface the smaller tire is more effecient, leaving out rolling resistance. I personally have an endurance bike with big tires, high front end, high end tripple gears, flat bar etc but i do not race, i value comfort. If you race then you will notice a sizeable pedal torque effeciency difference between a race & endurance bike.
Not to focus on those particular makes and models listed by the OP as it would be difficult to show the differences using those specific bike models.
Yes, a race bike is characterized by riding position. Traditionally with a slammed stem - steerer tube cut short and long stem angled downward. Seatpost set to near longest limit. Narrower width bars. Head tube is short and head angle steeper and a shorter rake fork making for faster more responsive steering. Tighter rear angles keep the rear wheel neatly tucked.
Modern race bikes are now showing more consideration to aerodynamics. Early aero bikes being super stiff but latest ones designed with more compliance and comfort. But they have a pretty focused look about them and may not be the ideal tool for the 90% of the riding you do.
So in summary, a race bike is a combination of the bike and riding position. Bear in mind - the ideal race position may not be ideal for you. It's a pretty extreme and brutal riding position but maximizes for all out speed if you can sustain that position.