It's mostly cosmetic. Different woods have different densities which can affect overall bow weight and limb weights. Heavier limbs will theoretically shoot a little slower but you aren't talking about much, just a few FPS but nothing of any real world significance. More mass in the riser will make it feel more steady in the hand and during the shot cycle as compared to a lighter riser. All that said, 95% or more of performance comes down to simple design/geometry, overall mass, draw weight, and to a lesser degree limb core type. So, if you like light bows pick something that's known for light weight like maple or if you want the stability of more mass go for something more dense like ironwood or cocobolo. Other than that it just boils down to aesthetics.