Interesting yet non-scientific survey by GCN of pros and the type of bike they would prefer – aero or light weight.
The results were 50/50 but more interesting was that some of the really big hitters opted for lightweight (Sagan, De Gendt).
Just one more thing to make you go hmmmm. 
I think how that aero or light frame is designed and constructed is way more important than how the industry is trying to sell us aero v. light weight. At least that’s what I hear when I talk to good riders. It’s never a grams of drag v. gram thing. Always a handling and a feeling component that is important.
Unfortunately those getting into cycling probably feel drag v. weight are the only things to consider and justify buying something based on those two parameters. I hope not, but, that’s what I interpret from what I hear people talk about around the water cooler and on forums like this.
I’ll admit I drool over some of the Aero bikes but I’m a little bit of a weight weenie deep down.
I feel like aero makes the biggest difference when you aren’t in the peloton, such as a breakaway. So Sagan wouldn’t really find benefit in that. But how much does aero affect the sprint? They are only out in the wind for a short amount of time. I guess it comes down to whatever feels better for someone like him .
Sagan won P-R with a 50km breakaway.
But I’m not sure he’d choose to ride a lightweight on the cobbles.
It’s more of a global thought process
My vote would go to light over aero simply because I think weight makes more of an impact at the speeds and type of riding I do but pros running for long periods at say 35+ km/h then aero gains may start to have a noticeable impact. Varied lower speed riding over roling terrain or significant amounts of climbing in my mind would favour a lighter, faster accelerating bike
So the real question is: is “lightweight” synonymous with more comfortable? Are these pros really saying that over 4 - 6 hours, if I’m sitting in the bunch, I feel fresher when I need to go if I’m on a more comfortable bike the whole time? My completely unscientific guess is that this is the reason. If you are sitting in the bunch, then having a more aerodynamic bike isn’t a bigger advantage than being less beat up at the end of race / stage.
I would guess that is probably it, lightweight may just be more comfortable for the amount of time they spend in the saddle every day. I wonder how close you could get to an aero bike if you used a more classic frame and added some deep section wheels and aero drop bars.
Plus fully internal cables to clean up the front end.
It would be a very interesting experiment to mount aerodynamic sensor (like one of these - Eurobike Aero Roundup: Notio, AeroLab, AeroPod, Swiss Side and other tidbits! | DC Rainmaker) on pro riders bikes, and see what the trade-off is between reduced drag of an aero bike vs. reduced power from a lighter weight bike. To say nothing of the comfort factor.
If you are in a peloton most of a ride / race, I wonder what the real trade-off is between aerodynamics and weight.
Was it when they talked to the Specialized wind tunnel guys on the podcast that they talked about the decisive moment in a race being most important for the pros? Like for a climber, even if there is 200k of flat and raw calculations show that an aero bike would be better for the whole stage, if there is a 10% climb where the actual selection is made, then a light bike is better.
For someone like Sagan on a normal sprint stage the decisive moment is usually the sprint itself, so the aero bike might be better, but for some stages he contends on he wins because he’s one of the few climbers that gets over a late climb, so a light bike might be better for a stage like that.
Dollars to doughnuts this is it. Lightweight bikes aren’t really making compromises because they can beat the minimum weight by so much. They get 80-90% of the aero, hit the weight target, and are still able to make considerations for ride quality.
If you aren’t going to have your nose in the wind all day I can 100% see why they would make that decision. Also if they have to burn 10-15 more watts in the peloton that doesn’t ruin their day or week.
I still think I would go with the aero frame if I was buying today because saving those watts over the course of a day will make a difference to me.
Aero is everything! Most of us would be better served by aero gains for our day to day riding & racing.
I always pick an aero bike. I think everybody else should always pick a lightweight bike.
If I get in the mountains no bike is light enough to save me! 
Make gains in both. Drop weight on the engine. Get an aero helmet. Find a lighter weight set of aero wheels. Internal cable routing on a lightweight frame. Etc. I think find a good balance unless you know theres only one use for a certain bike/setup. Many of us do lots of different riding on one or maybe two bikes, so I just try to get low hanging gains in both weight and aero. That said, am primarily concerned with the horsepower Andy weight of the engine.
I think it’s a question that’ll never be fully answered. The World Tour pros have a selection of bikes to choose from and will take their pick depending on the race. An interesting fact is the bike that has been ridden to the most victories in the pro circuit is the Tarmac, a GC bike! Another interesting point is that the current Tarmac is as aero as the original Venge.
Wheel choice is possibly more critical than frame selection. Cav was famously complaining that he lost a sprint because he had 404s rather than 808s on his bike.
I live in a hilly area, so lightweight bike every time. Also, the archaic UCI rules don’t allow the pros to ride the light bikes we can! No point in getting an aero bike if you can’t get into an aero position. It amazes me how many of the guys I ride with can’t spend any sustained time on the drops! Get a bike that fits and practice your aero position. Chad reminds us regularly as you know!!!