The word advantage / disadvantage has seriously muddied the water imo. This has been one of the most difficult topics to translate with thumbs. And that horse is definitely dog food now! The bottom line is…
you get 5 extra levels.
That have extremely negligible differences!
Which can vary…!!
If you want to focus on these minute differences then, I recommend you run the calculations case-by-case.
Sometimes the 5* is ever so slightly stronger after.
Sometimes the 4* is ever so slightly "buffed" more but still weaker than the 5*
The important caveat is that we work within the confines of our roster!
However…
All our rosters are evolving!
If you only have a couple legendary then it makes sense to go nuts on your rare and epic. But then you pull C.Magni from the costume chamber! Hurray and boo-hoo at the same time 
Also…
Player 1 believes he can’t achieve top 1% without max’d teams, and intends to only level rare and epic to get those rewards. He won’t touch his 5* until he has 10 of each lower rarity broken.
Player 2 knows he can achieve top 1% and intends to focus only on his legendary champions.
Both players receive a 5* aether from the same tournament.
Which will feel extremely elated?
And which will have very nice wall art in his inventory?
Just a bit more 
for 
As far as winners go… uh…
let it play out. 1 year from now ask again.
I doubt I’ll be here but… the answer about LB effects will become more clear over time
I don’t know for a “fact” lol
But my guess is… the patient players are going to have some silly defenses and there will be a flood of tears in the forum. A few extra dumpster fires. And even more players who just feel tired when they play