ENVE Here.
Thought we could comment on your question and hopefully help you make the best decision. 5.6 vs. 4.5AR. But to answer your question directly, we’d recommend the 4.5AR if you’re going to primarily use the wheels on your road bike, assuming your P Series has clearance for a tire that will measure nearly 30.5mm (the new Pro One 28 measures slightly smaller than the last generation).
Pros of the 5.6:
Faster than 4.5AR in strictly aerodynamic terms when running a 25c tireNo tire limitations, you can run tubeless, or non-tubeless tires - However! Tubeless will be a better experience as the tubeless design does make installing and changing inner-tubes a bit more challenging. It’s not one of those impossible feats, rather you just have more stuff to fit in a smaller space.
Pros of the 4.5AR:
VersatilityComfortHookless - There are many misconceptions about hookless. While it is true that some tires are not compatible, the vast majority of high-end tubeless tires from reputable brands are. Tubeless tires are getting better every year and the latest round of tires coming from most manufacturers are excellent. Check out the “Tire Compatibility” tab on the 4.5AR product page for the list of recommended tires. (Side note: The latest Specialized S-Works Turbo RapidAir passes our testing and will be added to the approved list soon. This is an example where ENVE worked closely with Specialized to establish compatibility w/ the AR Series rims.) The reason why some tires don’t pass our compatibility test has to do with the stiffness of the tire’s bead. If the tire’s bead is too soft, and there are no bead hooks on the rim to catch it when it stretches under pressure, the tire may blow off. Not good. Consequently, tubeless tires were never designed to establish a seal on the rim’s hook beads - rather, in a tubeless system, the tire beads need to achieve a seal on the rim’s bead seat diameter (that shoulder on either side of a tubeless compatible rim’s tire bed/channel). So, if the tire is stretching off that shoulder, the tire isn’t achieving a proper seal, and therefore becomes prone to burping and at worst blow off. Today, most manufacturers making tubeless tires have figured this out and are making great tires that will work without reservation on a hookless type rim. So why hookless? It is the best way to make a tubeless rim. It allows for the use of machined hard tooling to create and establish consistent and precise bead seat diameters on the rim. Aerodynamics - The 4.5 AR is very fast and by far the most stable wheel in our lineup as it relates to crosswind stability. While not as fast in a straight up drag race against our 7.8 or 5.6 in the wind tunnel, it is only a couple watts off the mark. Rolling resistance - The watts lost to the 5.6 can easily be made up for in RR if the surface is rough, and a fast 28c tire is chosenNearly everyone at ENVE rides and races either a 3.4 AR or 4.5 AR, our lunch rides frequently average 25mph plus… the AR w/ a 28mm tire is simply confidence inspiring.