Even the 10 ride purchase gets you the buck-thirty price. I usually buy 20 rides a pop for $26, but 10 rides for thirteen bucks is still an available option.
A metrocard givesa 15% premium (when you put more than eight bucks on it), and you get dinged the the face value of $1.75 per ride. Ten rides is $17.50 on its face value, for which you're paying thirteen bucks on the PATH fare card. That's just a shade under a 35% premium. A PATH farecard twice as much of a discount as a metrocard, over the cash pare.
As far as I see, the only reason to use a metrocard is if you're not a regular PATH rider, but have a pay-per-ride metrocard with some money on it. That way, at least you get to take advantage of the 15% premium value of a metrocard, instead of paying full fare for a single-ride PATH ticket.