In general, the density of Maple is higher than Alder but the ranges overlap so you can have a maple body that's lighter than an alder body.
http://www.engineeringtoolbox.com/wood-density-d_40.html
Solid Density (lb/ft3)
Alder 26 - 42
Ash, white 40 - 53
Ash, black 33
Balsa 7 - 9
Bamboo 19 - 25
Basswood 20 - 37
Ebony 69 - 83
Mahogany, African 31 - 53
Mahogany, Cuban 40
Mahogany, Honduras 41
Mahogany, Spanish 53
Maple 39 - 47
Redwood, American 28
Redwood, European 32
Rosewood, Bolivian 50
Rosewood, East Indian 55
Spruce 25 - 44
Spruce, Canadian 28
Spruce, Sitka 28
Walnut 40 - 43
Walnut, Amer Black 38
Walnut, Claro 30
Walnut, European 35
Zebrawood 48