As a general rule, triple pane may not be the best alternative if your promary concern is sound reduction. In fact, depending on whose window you are installing, a dual pane could out-perform a triple pane in sound attenuation.
Stopping unwanted sound thru a window is determined by three things – mass, stiffness, and damping. Increasing the mass of any object will have an effect on sound transmission. Increasing the mass of a window (making the glass thicker) will increase sound attenuation, but not nearly as much as might seem obvious.
It really isn’t possible to increase the stiffness of glass, so that leaves damping the sound as the only remaining variable.
Glass has very little damping ability, but when putting a layer of a more viscous material between two of lites of glass we substantially increase the units ability to dampen sound – thus the advantage of laminated glass. In fact, a single pane of 1/4" laminated glass consisting of two 1/8" lites with the plastic interlayer actually has as much sound blocking ability as a 1/2" lite of monolithic glass.
Two things you want to look at when comparing windows - whatever glass is installed - first, how tight are they when closed? That is huge. If the window doesn't sit very tightly in the frame, then you will have sound getting thru the unit.
Second, how "heavy" does the frame feel to you? They will probably feel about the same I suspect, but any flimsiness or anything else that may make you wonder about how "sturdy" the window is, can affect sound.
Next, which ever window you decide to choose, it MUST be installed correctly. I would venture a guess that better than 90% of all window problems involves installation, and if you want to stop sound from penetrating the unit, then installation is vital.
A fixed, versus an operating, window is usually going to give you much better sound attenuation. However, having a house-full of windows that can't be opened is not very appealing to most people, so again we come back to the earlier comment that the window MUST be tight when closed.