If you're looking at the new Aero bikes from Scott, Specialised, Trek etc, then you're also getting a lightweight bike.....you're also shelling out plenty of cheddar!
Many hours = comfort and setup / position on the bike.
Are you racing your bike?
You can "buy speed" on a bike up to a certain point, but why not spend that $$ on a training program, nutritional advice, working on your FTP and making your self as light and as powerful as you can be?
Me - I would go for lightweight over Aero, and as Winky says, I would focus on my own weight and not the bikes too much.
I think Wheel / Tyre Combo's are somewhere that gains can be made in your bike setup - have you considered this? Also a key to comfort for long days in the saddle.
One of my good friends rides a Steel Cervelo Prodigy, is over 50yrs old, has been riding since Adam was a Cowboy..... still rips the legs off all the young Punks riding Carbon Aero, Di whatever, Strava blah, blah blah.....yeah, I know we all have one of these stories, but he is the real deal....hairy legs and daggy old kit that needs to be thrown out
There was this guy called Eddy who once said "Don't buy upgrades, Ride up grades"