ND filter question...

[MosaicW]

I'm looking at buying a couple ND filters and I'm not sure whether 8x or 4x would be more beneficial. I plan to use them mostly for outdoor portraits, or to try some better water-flowing-stuff, etc. I wouldn't mind getting one of each to compare, and I'd love to have one for each of my 2 lenses (an 18-135mm zoom and a 60mm macro) but I'm not sure which to get for which lens. Would it make more sense to have the 8x for the 60mm lens, since it's usually shooting wide open, and the 4x for the zoom lens since it's more likely to be shooting with a smaller aperture? Or is there a flaw in my logic...?

Or am I nuts, and don't even need to bother with a 4x... or will an 8x be pitch dark unless I use really long exposure times?

Help! Never tried one out before.

Jen