Differences between endurance and aero are very subtle, we all agree. We are also talking about head tube angle, fork rake and offset, chainstay lenght and downtube angles, etc. It all translate to: a more stable and compliant bike for endurance, vs a slightly stiffer more nervous if you are talking about a full on aero race bike. The aero design, early on resulted in very stiff, quite un confortable rides. That now has been fixed and improved.
Now on the gains, all things being equal (position, components and wheel/tire, etc) yes an aero frame may be more efficient but that is a fraction of a fraction. Merckx in the 90 identified that body drag accounted for 75-80% of total drag leaving 20-25% for all things bike. Getting an aero frame won’t reduce this to 0, you will agree. The best of the best frames will cut 1-2% of total drag. At 300W we are talking 3-6W (not accounting for wheels).
This is like all things bike, a lot of marketing pinpointing your desire to have that n+1 beautiful sleek road machine with that “je ne sais quoi” which for sure will make you crush you Sunday ride buddies on that last sprint to the coffee shop. We have see that when everyone were weight weenie and were spending ludicrous amount of money to save grams on stems and seatposts.
Now, don’t dispair, we are all looking at those magnificent machines and drool. On the other hand, you can reap significant gains by training in that aero position @chad reminds us to get into, you will shave a significant amount of wasted watts with an investment of 0$.