Like almost anything having to do with growing plants, it depends. Are you growing wine grapes or table grapes? What variety? Where are you located and what is your soil like? How do you plan to cultivate--what equipment do you need to run down the rows and how big is it?
In general, table grapes produce more tons of grapes per acre than wine grapes, which are usually restricted in production to improve quality. As a practical matter, you will probably need the rows at least nine or ten feet wide to drive your
tractor down them, although there are specialized narrow tractors for vineyards and even tall tractors that are used to drive over the top off grape vines.
The "y" type trellises are used most commonly for table grapes or a few varieties of wine grapes that are very vigorous and best grown on a taller trellis. I grow wine grapes using a double cordon system, but I'm just a beginner. There's a lot to know.