Which type of sliders is easier to use: horizontal or vertical?
I say, it certainly depends on what the slider is intended to do ... and how often, not least.
Vertical sliders cater to the use of the scroll wheel on the mouse. This has the benefit on quick, easy, and intuitive interaction, although some amount of precision is typically lost.
Horizontal sliders, on the other hand, are commonly (cf. mariocatch's answers) used for adjusting properties. The advantage is the possibility of a high control density (a vertical slider - and it's label - can consume the entire screen width and still only require the equivalent of a single line of text).
If you're talking about sliders per se, and not scroll bars, then to the best of my knowledge the vertical motion is somewhat physically easier. You're just contracting your fingers, in the most natural way there is, like you're grabbing something or making a fist. The movement of the wrist left and right is "less natural", if you could call it that. The wrist is built to move primarily on the axis that goes down (where the palm faces) and up - even in a common person it has a range of about 180 degrees, while its range sideways (in the direction of the thumb/pinkie) is much more restricted and uncomfortable - about 90 degrees total. In addition, the contracting fingers move the vertical slider strictly on the vertical axis, while the sideways motion of the wrist doesn't take place on the horizontal axis, but describes an arch -- unless you're moving your whole forearm.
That being said, usually horizontal sliders aren't as long as to make the difference felt, and you can compensate for the arched motion of the wrist by using the fingers. I don't think there's a meaningful difference in everyday usage.