ND filter for an an amatuer --- NDx400 or NDx8

Lookin for an ND filter so that i can use slower shutter speeds to shoot waterfalls and to record movement in subjects such as clouds, seas,grass,etc.So which one should i go for --- an ND 8 or ND 400 ?? Am just an amatuer when it comes to photography.Infact i only started shooting 7-8 mths back when i got my nex-3. Do you think ND400 will be difficult to use or an overkill for someone with my skill levels?
Any pros and cons one has over the other ??