I guess it depends on whether you prefer 2D games to 3D games. Keep also in mind that mulitplayer is very expensive on the GBA since everyone needs a system. So if multiplayer is a big deal to you get a Cube. The Cube also has mostly new games while the GBA has a lot of ports you may have played before. Finally when the GB Player comes out you can play GBA games on the Cube but you can never play Cube games on a GBA.
So I say get a Gamecube and later a GB Player.