This question is usually the first thing people ask about.
The Slider is slightly shorter and comes with a Schmidt Easy Flow 9000 medium point with black ink and will fit all other "Parker style" refills.
The Glider is the slightly longer version that comes with a 0.38mm Pilot G2 with black ink refill. These are a little easier than the old pens to allow refills to fit and most require no modification at all for many different refills.
Both pens fit well in almost all hands, though if you have larger hands you might be more comfortable with a Glider. Sliders tend to fit better in shirt pockets without sticking out.
Overall there is much less need to modify refills on these pens than previous pens. If you do want to fit a few of the refills from the Glider list in the Slider check out the video below. Sliders will include the Glider spring.
The main piece that makes these pens really interesting is the bolt action. I use a shape unlike any other pen out there. It is one fluid motion to advance or retract the refill. Because the surfaces of both the interior part and the inside surface have been carefully machined to be extra smooth, the action feels amazingly satisfying.