Solid headstocks are the usual option for both guitars and ukuleles simply because slotted headstocks were/are traditionally used only on instruments with 12 frets to the body necks. Being easier to construct and finish makes the solid headstock the usual standard and slotted headstocks incurring an up charge.
~ Solid headstocks are easier to quickly restring.
~ Solid headstocks have much more area to explore custom inlay designs.
~ Much larger variety of tuner options to choose from over the comparatively limited (but stylish) slotted variety.