Shafts have other characteristics than the stiffness. Based on which section of the shaft will flex the most, they can dramatically affect the launch and dispersion pattern. The m series in the 915 is a higher launching shaft than most. A factor we can't determine as we are not physically fitting you is how you transition at the end of your back swing.

Stiff versus regular is only important in comparing 2 flexes of the same shaft. One shaft's regular can be another shafts stiff, there is no standard across shafts for flex. Not to mention that Titleist driver heads come in 4 different lofts so it can be like comparing a 5 iron to an 8 iron.

Long story short, you at least need to hit both to decide for yourself or work with a qualified fitter or pro shop to find the characteristics of your swing for the appropriate loft and shaft combination. When I got tired of buying and selling/returning used drivers, it was a whole lot simpler going to a pro shop and getting fit for the driver that worked with my swing. I could have kept it for 4 years, which was a better investment than the 6 drivers I didn't keep in my first year of getting into golf.