ND Filter Help

[jonnypb]

Would like to get some/a ND filter so I can do some long exposures 20-60secs depending on light.

I'm getting the Sigma 10-20 and Tamron 17-50 which are both different filter sizes. I'll probably use them on the Sigma most the time so should I just get a screw on or a holder with slots so if I want to use it on the tamron in the future I could

Any advice on a good setup would be appreciated

Thanks