In my experience, if you're running a unit of 10, it's your choice. Javelins are easier to use, but their shorter range is limiting.
If you're looking to max out a unit of 40, use the boltspitters. The +2 to hit for the big unit makes an amazing difference, and you'll get work done.
Either way, give them bucklers. It's still a 6+ save, but now you get to ignore rend -1. If your skinks get into close combat, you'll want to retreat more often than strike, so the clubs aren't particularly useful.
Again, these are just my opinions -- I don't purport to have any amazing tactical insight, but this is what I've determined from my experience.