Holloway is the clear favorite for a reason but when you factor in age, including ring age, I think Conor sitting out taking PEDs is more of a benefit. That's 5 years without taking damage to his brain.
Guys like Holloway are near that Nate Diaz age with serious ring wear. They've been in wars. Conor's been in some tough fights and the effect of the leg break remains to be seen. But don't count out the possibility that Holloway's ring age might be significantly greater than Conor's, and that could potentially be a factor in the fight.
See Conor's statement here where he addresses ring age