William J, There are two big differences, the 915's are LONG AND STRAIGHT!!!! Get fitted and you won't be disappointed. steve b

William:  All depends on the setup.  Go see a certified fitter and get the right combination (shaft and head).  The 915s a very good club and with the right fit, you will find them to be a very reliable club and an improvement over the 913s.  Titleist now offers three heads:  D2, D3, D4.   Cheers, Chuck...... 

And the winner of the other 2 majors used 915s. and FYI it isnt the equipment that is giving them the victory

I had the 913F and was using a png G25 Hybrid. I bought a 915H and immediately noticed large difference in accuracy and feel. So I decided to buy 915F . Since I had 913 and 915, I went to the range, using same set-up hit several shots with each. I noticed the 915F was about 10 yds longer, but more importantly, probably 5-8 yds straighter. I have not switched out my 913D2, because really didn't get enough change to merit the switch. To me, my biggest issue is feel and confidence. With the 915 both those are great! Good luck.