The Wii does not support 720p.
I'd say use component for Xbox 360 and SCART (since you live in Europe?) for Wii. Best would be component for both but I doubt your TV has two unless you live in the US. Xbox 360 supports optical (only way to get true 5.1 surround sound) and Wii only supports stereo (red and white RCA cables).
//Danne