Hmmm, 50-50, either way the time will be the same. I did the drive this past Sat. but went the freeway route because the air show was going on in Beaufort; according to a newspaper article read after the fact this was a very good decision! But in general, Saturdays this time of year are pretty reasonable as far as traffic goes. Since it will be daylight, I'd second what Disneyaholic said and go through Beaufort for the scenic drive. Be aware that there is a 22 mile stretch that is one lane roads with 3 passing areas. The little shortcut you can take is after you pass the airplanes in Beaufort: turn right on the 802.
I-526 (west?) until it dead ends
right on 17 south
21 south toward Beaufort
right on 802
right on 170 (note: do not speed too much after this point!!!)
ramp to 278 toward Hilton Head