This is a very "loaded" question, pardon the pun
OK, let me do my best to explain. Ladder line is a "balanced" transmission line. Depending on the dielectric property of the insulator and the spacing between the conductors, it presents an impedance that is different from the resonant impedance of your antenna element. Ladder line is typically in the hundreds of Ohms. It's been a while since I built a dipole with balanced feedline, but 300-450 Ohms sounds about right.
When you feed a resonant antenna with coaxial (unbalanced) transmission line, all you really need is a "BALUN" at the feedpoint...this is because the antenna is a balanced dipole and the feedline is unbalanced..requiring a transformer to accomodate the unbalanced to balanced transition. This is debateable BTW, as many people build resonant dipoles with 50 Ohm coax without baluns and they work just fine.
At any rate, your balanced transmission line is theoretically a better method transferring power to the antenna feedpoint because it is balanced, and the cool thing about balanced conductors like this is that common-mode currents tend to be nulled. However, if a conductive material or structure is in proximity it can cause an imbalance. Any imbalance will cause coupling and this advantage is no longer seen.
Balanced feedlines are typically used with DOUBLETS, that is, dipoles that are used on multiple bands. There is a reason that amateur radio bands, for instance, are multiple wavelengths of each other. A doublet cut for the longest wavelength band intended will work well, with a tuner, on the other amateur bands that are higher frequency but wavelength divisible.
If you are building a single band antenna, the advantage just mentioned will not be realized with balanced line. You are better off in reality with 50 ohm coax because tuners create a certain amount of heat loss. With a resonant antenna and 50 ohm transmission line the tuner is not needed. Now, you don't have to worry about coupling the antenna and causing the common mode currents and radiating feedline.
The antenna tuner is a capacitive-inductive network with, at least, a variable capacitor...the impedance mismatch of the feedline and antenna feedpoint can be "corrected" with the tuner...if the reactance of the antenna is more inductive than capacitive, the capacitor can be tuned to add capacitive reactance and peak the power transfer, much like a resonant parallel tank circuit. If the antenna is more capacitive than inductive, the capacitance of the tuner can be reduced to the same effect.
Again, the best feedline for a resonant dipole is coax and the best solution for a multiband doublet is open wire or balanced feedline with a tuner.