Here is a link to a thread about grip shape. If you scroll down you will find a diagram comparing Head, Dunlop, Wilson, and Prince grips. IME Volkl grips are more rectangular than head grips.
Grip shape is all about personal preference. There are people who rave about Wilson's grip shape, but I personally prefer Head/Volkl's grip shape.
I once lost a match because at 5-2 up in the last set, my Wilson racquet's grip had been digging into my fingers and on an overhead it caused a blister too burst. I couldn't hold the handle anymore. I have never been as angry as after that match. I was seething. I was crushing the other guy and after the blister, I tried to tape it, but it was open skin and hurt like hell. There was nothing to do about it, I coulnd't play a single shot anymore.
Obviously this also happened because I hadn't played in a while and my skin had become too soft. But still, loosing a match because of a blister.... not fun. This has never happened to me with Head grips, they suit my hands a lot better, especially for volleys, where I feel the Wilson grip is too square and just digs into my hands.
"Most of us believe in trying to make other people happy only if they can be happy in ways which we approve." Robert S. Lynd