At that point it's more about the tempo in your transition. If you're quick, so with the stiff, if you're smooth, the reg. will probably do!

It's more than just swing speed. It's your transition and overall swing that will affect what works best for you.

It's best to try as many as you can, a fitter usually helps as they know about shafts bing offered and what might be best for you.

It's also about what's most important to you. One shaft/flex may give you more distance and another may give you more accuracy...

5.5 is stiff, I thought under 90 mph was regular shaft users - think you would be stiff and over 105 x stiff

5.5 is a light stiff with would be a no brainier for its just not a stock shaft for Titleist. I'd like to Try the bassara stiff hopefully the 917 will have a bunch of stock shaft options

In general, I agree with "TitleistUser" above.
If you are close to a Titleist Thursdays event, I highly recommend it. As "B.A." said, the Titleist Thursdays fitters know the product and can quickly put you into one or two shafts that will work for you.
In my case, it wasn't shaft flex but shaft weight. My old driver was too long and approx. 50 grams. My dispersion was all over the place. After the fitting, I am using a 74-gram shaft and the dispersion is 1/3rd of what it was. JAT.

I just had this dilemma when being fit for my 915 D3 driver. I had always used a stiff shaft because for the longest time 105 mph was the defining number. I found out that because of shaft technology, torque around the club head is better controlled. This allows for the faster swing speeds to use lower flex shafts because the head does not rotate as much. In essence, you can create more power through the swing plane with far less club head rotation due to the torque of the transition and potential energy in the shaft leading to a longer shot.

So long story short, 105 is absolutely fine with a regular flex shaft but obviously you should get fit and test out the different types out there.

Probably stiff however I have a similar swing speed and I am a X-Stiff according to TrackMan. It depends on your swing characteristics and the shaft.

Playing XP105 S300 shafts on my AP1s. When fitted for my driver he put a Aldila Rogue Black 70 S for my 93 club head speed and ball spin 2000-2500. Of course I am an old guy going on 70.

In my opinion I would say this....

Use the shaft that gives you the best results. It really shouldn't matter what the letter says... it's the "end game". Somebody already mentioned other factors and they were spot on with saying that it's a lot more than swing speed. Tempo and transition are as important. When in doubt get to a fitter or a demo day and see the results in "real time".

Alex

Much like you, I'm right in between. I have been using the Project X Blue 5.5, as I think it offers the best middle ground. It's nice and stiff in the tip and butt, but I can feel it.load so easily. I think it's a "best of both" shaft. Sometimes with softer R-flex shafts, I have a tendency to lose control if I really go after one, the PXBLUE has changed that, it performs well on a nice easy tempo swing, but also lets me get after it on long holes. I'd like to try the Rogues, I've heard really good thingstuff, but for now, I'll stick to the PXBLUE.