SX3 OR X3:
If you want the best performance, and can afford it get the SX3. The sealed, 1.33 HP SX3 BLDC motor has about twice the power of the X3 motor, and it can carry this torque into much higher RPMs. The Novakon NM-35 uses a variation of the same motor, and it has a top speed of 6000 RPM with still good torque. The SX3 uses quiet belt drive rather than noisey gears, so runs much quieter. By adding a 1:1 motor pulley/belt, 3500 RPM is possible. The SX3 has the motor INSIDE the head, not hanging off. For CNC, this is important, not only for flood coolant protection, but because you can bolt all kinds of accessories onto the clean head--like a super high speed spindle, Surface grinder, Power drawbar. When you CNC an SX3, there is a much lighter head for the motor to have to lift.
The only thing the X3 can do BETTER than the SX3, is have Mach3 easily control spindle speed. Someone WILL come up with a way to do this soon though--And meanwhile there are work-arounds and you CAN control motor on/off with Mach3.
Grizzly has the lowest cost SX3 available--and they have just lowered the price $100 from last year. You will pay more for an SX3 than for a X3, but IMO, you get what you pay for in the very powerful BLDC spindle motor, quiet belt drive, compact head design, tachometer and the possibly useful 90 degree tilting head. There are other bells and whistles, but most of them are discarded during CNC conversion.