the seam would be easier in some ways & give a person options as far as where to put several tie outs along that horizontal line.
it would be much stronger also. the tie outs not only serve as "pull outs" that make the tarp much roomy-er & make it easier to get in & out w/o touching the tarp when it gets wet on the inside, but they are also helpful as extra security in high winds.
this will be just like the seam you see in the picture in the above post except higher.
the seam in the picture was for adding a side wall... but that's another story<g>.