If you can, get the 6 x 24 rectangle because that one is long enough to cut the folded width of fabrics up to 48 inches wide.
Again, it depends on what you are planning to do.
I cut most of my strips on the lengthwise grain - I don't remember the quilter/author that I learned that from - that way at least two edges are more or less on grain.
If you are planning to cut strips from selvage to selvage (crosswise), definitely get the 24 inch ruler.
My opinion.