The 'Pro' models within the Talon series are really decent backpacks. As Sara mentioned above, the first consideration for a backpack is 'Fit n Feel'; after that, it is all about durability and craftsmanship, ease of adjustability (fine tuning adjustments with the hip and shoulder harnesses on the fly), usability, and personal taste factors related to colors, loading (top or panel), and size and placement of pockets and do-dad holders.
My preference is that a backpack needs to hold everything inside the pack. . . I do not want to be forced to dangle shoes, underwear, sleeping gear, etc. due to lack of space, unless I choose to dangle stuff. And I want the bag large enough so that I am also not needing a shoe horn to compress everything as tight as I can.
BUT, if I have the choice between a backpack that is exquisitely comfortable to wear and carry stuff with, but is forcing me to dangle some bits, versus a backpack that is spacious enough, but is fussy to carry and never lets me forget I am have a load on my back, I will choose to dangle bits and enjoy the comfort of the carry.
So if happiness is 20 liters of carrying capacity, and the Pro Talon fits and feels good, you have a winner. PLUS, it can be carried on a plane with no questions asked. . . well, I guess airport security will ask and demand to know how you got that guitar inside the pack, and then confiscate the guitar because guitar strings can be used to garrote the flight crew and hijack the plane to Cuba, but somehow overshoot that island and land in Haiti where the Haitian police will think you are nuts because Jamaica has better beaches and cute tropical umbrella drinks and is just over there to the left across the ocean a little bit . . .
I dunno, maybe the Talon Pro 20L is too much of a risk for international misunderstandings and diplomatic angst. At least Haiti doesn't have nuclear weapons.
All joking aside, it sounds like you have a good backpack that will meet all your needs.