ND Filters

I have been researching some ND filters, as I have never used them, and looking to gain insight from others in group. I have seen some that offer the ability to adjust by a slide on the side, and some that also include a CF filter embedded. Others recommend an outside attachment that you can slide the filter choice into versus a screw on or magnetic filter. The other one suggestion I ran across was to get a 6 and a 10 filter as good starters. Please share any relevant suggestions you might have, as they would greatly be appreciated and more likely than not, cost affective as well! Thanks!