I say no to any sort of bow, but the sling should have improved stats. In the hands of a proficient user a sling is a terribly deadly implement.
My only justifications for my position are:
I like the current balance of the Druid weapon list
The weapons on the list now all feel great for druids as they are either relatively easy to make or repair with little support or very durable and have heavy symbolic significance, though I'm a little iffy on scimitars truth be told.
Bows don't fit this as all bows save for, perhaps, the simplest hunting types are very sophisticated artifacts that require a lot of care, time, and equipment to maintain much less create. Think of the amount of stuff you need to fletch arrows, for instance, arrowheads, good feathers, good shafts, twine, glue, and all the equipment and materials needed to make the above and put it all together. A spear still takes a lot of work, but you're a lot less likely to have it lost or broken. Bows are generally for people who hunt in the grand style. Unless you're a savage barbarian or horse nomad. Peasants aren't likely to use them, and I think of the Druid as a humble sort of fellow. At least in his use of weapons and technology.
X-bows are simply too techy and new and expensive.
In specific settings I could see how bows or x-bows might work, but I like them absent in the generic version.