which do you like better?

Use some blue painter's tape to cut several 6" long skinny strips of tape, and put them in position. Then stand across the room.

Ditto w/ the 8"

Just remember that the blue tape will seem more prominent than your pulls actually will.

I don't think it matters.

With only 27" widths, 6" is enough; it won't look undersized.

But if you like the extra drama of the 8" (you'll have only 4.5 inches of blank drawer face on either side), that's a perfectly good choice as well. Just more dramatic, which some of us (me!) like.