I did not spend much time looking at these machines because I already have an SX3, but I believe the main decision factor is that the SpeedMaster has a bottom end of 8000 RPM. That means you can only use it with very small cutters (perhaps 1/8 on steel and 1/4 on aluminum) and for engraving.
If you are looking for a general purpose machine the X4 or X4+ is a better choice -- but then your small cutter milling and engraving will take much longer. Not sure what the difference between the standard and the plus is.
If you have the money get the X6 (I would). However, what you described should be handled easily by the X4 unless you have a lot of product to make.