Lukitoh,
If you have the choice of the 550 or 740, I say take the 740 man...!!!!! The 740 is the 550 and then some.. You get 4 variations, dot matrix screen, way better voice quality, good speakers, draw bars, vocalizer, Master EQ, modulation wheel, and just an overall better keyboard..

Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.