Curved or flat, well the answer regardless of what any one can use or handle is curved. A curve is a stronger and safer structure than a flat one, and in the case of the fingers the curve takes a lot of stress and strain away from the finger joints by allowing it to run through them rather than over them.
If the fingers are not curved then where is the bend coming from to get them to contact the fretboard? Answer...the wrist, with variations in bass height.
The wrist has to compensate for any lack of curve the fingers have because the flatter the fingers the more wrist angle is needed to achieve contact with the fretboard. With curved fingers ( varying degrees are personal) the angle of the wrist can be reduced thus reducing the stress and strain on them.
I is a individual thing to each player, but should never taught as such, it should always be taught curved finger. Yes i know there are players that play flat finger and handle it with great ease, but like i said it is an individual thing...exceptions to the rule do not prove the rule.
If in doubt or not quite sure about what i am saying about the relation in the angle of the wrist and the curve of the fingers then look at your own. Notice as you straighting the wrist, the finger need more curve, the joints come in to play more, the stresses, strains, energy, movement etc, are all going through the joints and through the joints in a chain reaction, all joints are involved. Put your hand palm down on a hard table/surface with the fingers spead out flat. Now press the tips into the surface...see how the top joint closest to the tips goes in on itself slighty and goes white a bit. That is the stresses and stains going over that joint, not through it. Put some curve in the fingers and it will go away because it is now working in line with the other finger joints. This will protect the joints and stop the likelihood of any joint going in on itself and causing damage.
That damage can take the form of wear and tear and may not show up until later life, but as i say it is an individual thing.
As far as tone goes, again subjective to the person, as for comfort..subjective to the person, ability to do either one, subjective to the person.... as you can see it is the players choice and if you have one, because not all players do, then it is curved fingers everytime....and that is in both hands for the exact same reasons.
