I presently have one of each, a stepper system CNC which I built, and a factory assembled servo CNC which uses the Gecko 340's.
As for the difference between the speed and accuracy between the two; there is none that I can see.
The stepper system utilizes adjustable microstep drives which I operate in the 10 microstep mode. Power supply voltage is 48 volts DC. I use precision ballscrews which are 10 turns per inch on the X and Y axis, and an 8 turns per inch leadscrew on the Z axis.
The servoed system utilizes Pittman servos and 250 count per revolution Pittman encoders. The power supply is 36 volts DC. The leadscrews are 8 turns per inch on all axis.
Both systems will easily operate at 60+ inches per minute. Somewhere above that, the servo system will sometimes get a following error and fault the drives due to endplay on the X leadscrew due to the method used by the manufacturer to control that parameter. (vibration over time allows the endplay nut to loosen.) No problems at 60 IPM and below.
I have had the stepper system 3 years, and the Servo system 1 1/2 years. There is not any difference in accuracy between the two systems.
With my experience with these two systems, if I were to build or purchase another CNC system, I would go with steppers if cost was a factor. I would go with servos, if noise was a factor. The servos run cool and are very quiet. The only sound is from the spindle. The steppers run extremely hot at 48 Volts DC, and the steppers whine at high transverse speeds.
I have only used my systems on wood, thin aluminum, and laminates, and use a Porter Cable laminate trimmer (30,000 rpm) for the spindle. (I also found it necessary to purchase an electronic speed control in order to reduce the spindle speed, especially on the metal and laminates.) Most of my cutting occurs at 30-60 inches per minute.
You have the right idea on purchasing the best components one can afford. The quality of components, and quality workmanship is the key to a quality machine. One cannot have both very high accuracy and repeatability using cheap components and / or shade tree engineering.
Just thought I would share my experience with you. Hope it helps you come to your own decision.