It works both ways and what the clerk told you was based on the same yardage as the thinner yarn - both would be about 2500-2600 yards. Also, by doing squares it takes more yardage than if you did it all in one piece or even long strips that you'd sew together.
If you use the same amount of stitches for each square with a larger needle, you use a lot more yardage, but you'll have a ginormous blanket. Or you could do less squares to make approximately the same size and that would probably take about the same amount with the other yarn, possibly less, depending on the size of the squares and the size you want.
If your squares are the same size however, then you would probably need less even with bigger needles because each stitch and row will be larger than with the thinner yarn and smaller needles.
sue- knitting heretic