I would avoid variable NDs... they tend to give color casts and/or uneven, splotchy effects... even the good (and very expensive) ones. Those things might be correctible in a landscape shot, but I would think more of a problem in a portrait.
I'd also suggest a correctly sized screw-in filter as that will allow you to use your lens hood, which is even more important once a filter is added to a lens.
On a sunny day, I figure you need 6 stops of ND (using the Sunny 16 rule, it's 7 stops to get to f1.0, but by using 1/200 you are "giving back" one stop).
A high quality ND such as a 72mm B+W 1.8 ND MRC(multi-coated) costs just under $160. With B+W and some other filters, each 0.3 of ND equals one stop (0.3 x 6 = 1.8). NDs are also sometimes refered to by their multiplication factor, where a 6 stop filter would be "64X".
You can fine tune for cloudy days, shade, etc. by increasing your ISO.
Or, if you needed a little more ND reduction for some reason, you could stack a weaker ND filter onto this one or use a C-Pol with it. Or you could use ISO 50 on your cameras.
In addition to the 1.8 ND (6 stops or 64X), other available B+W ND filters are 0.3 (1 stop, or 2X), 0.6 (2 stop or 4X), 0.9 (3 stop or 8X), and 3.0 (10 stop or 1000X, rounded off from 1024 actually).
Hoya and Marumi filters are also good, though I'd look for their multi-coated versions. They have somewhat different selection and naming convention than B+W. For example, Hoya offers an ND400 or X400 that they call a 9 stop filter (9 stops actually should be a 500X or 512X multiplication factor). Hoya also offers 1, 2 and 3 stop filters. I believe Marumi presently only offers 1, 2 and 3 stop ND filters.