<b>DPDISXR4Ti</b>:
Gotta wonder about the handle. I think I see something about a foreign Mercury there...
The difference between 3/8 and .325 is basically that the first will have fewer cutters in a given loop and they will travel at a slightly faster pace. Picture a circle the size of the engine stroke and then outside that another the size of the rivets in the chain when wrapped around a seven tooth sprocket. A 3/8 circle will be slightly bigger than a .325 circle (hence the greater speed of the chain), but the .325 circle has a greater mechanical advantage against it from the crank circle than does the 3/8, so more torque is available to pull the .325 chain. The .325 chain will also, all things being equal, stay sharp longer because it has more cutters doing the same work. It's also often smoother in use than 3/8. Sometimes faster as well.
The only real factor though is interchangeability with other equipment. If that's not a concern at all, then on a smaller saw like the 55 the .325 would be the better choice. You might also have the option of running an 8 tooth drive sprocket, which is almost exactly the same pitch diameter (chain around sprocket circle from above) as the 7 tooth 3/8, if the saw can pull it successfully with the greater "toothiness" of the chain.
Some makes of .325 chain produce a narrower cut than their equivalent 3/8, too, which would also go toward an increase cutting speed.
<b>Ryan</b>:
10 decibels greater volume is perceived as being twice as loud, so I don't think it would take an engineer to note the difference.
Glen
[edited: modified "circle" relationships]