I looked up the types and assume you're referring to the Hoya brand. The ND8 is 3 stops which is a good starting point. I couldn't find ND400 on the Hoya website, but did see NDx400 which is only 9 stops and not variable from what I could tell.
If the ND400 is variable it will be thick. It will probably vignette just a little on the wide end of your 17-55mm lens on a 7D.
If you have the money, a variable ND filter is certainly more convenient, but it had better be very good glass so it doesn't degrade the image. I use Singh-Ray and don't have any experience with Hoya, but I've read good things about it.
If I had to choose between a 3-stop or 9-stop non-variable ND filter, I'd probably choose the 9-stop because it would be easier to use a longer shutter speed rather than have to use a small f-stop and fast shutter speed with the 3-stop filter.