I agree with allthumbs about the big main advantage for a six string lap steel guitar being easier (and quicker) to make the
slide face an angle (for example 45 degrees to the strings) so you can make different chords. I think it might be slightly easier to get a nice slow wide vibrato as well. Don't forget though, you can always hold the slide over the top of the neck to get this same advantage on a regular guitar; Adrian Belew plays slide this way.
However, if you are *listening* to a particular lap steel guitarist you might want to check to see what kind of guitar he uses. I know a lot of lap steel guitar players (maybe most) use an eight string guitar with a sixth somewhere in the tunning which will automatically give you more possible chords. I also think there are ten string lap steel guitars which have even stranger tunning systems.