I have been using an SL66 with 50, 80, 120, 150 and 250 for many years and added a Hasselblad 555ELD with 180/4 recently, which I have not used much yet. First the SL66. Compared to a Hasselblad or Rollei TLR, this camera is very bulky. Even the 555ELD is more compact though it has the motor+battery housing. An unmotorized Hasselblad looks tiny in comparison to the Rollei, which is about the size of an RB67. I think the weight of both systems is nearly the same. The SL66 is heavier than a 500CM but the Rollei lenses are lighter than the Hasselblad ones since they have no shutter. So if you carry a large system (which is not what you intend, just wanted to mention), the Rollei could be even lighter.
The great advantage of this camera is the bellow, which allows you to focus seamlessly from infinity to close up range. I became so used to this that it is always a bit of frustrating when I have to stop at 1,3 metres with the Hasselblad. Definitely an advantage for portraiture, especially since you are after headshots. Adding a 21mm tube to the Hasselblad will help, but it is not as convenient.
If you should choose a Hasselblad, I would also recommend a motorized model. They are cheaper and the automatic film advance is so great for portraiture. I´ve never been a fan of automatics but it really helps since you can shoot in short sequence and you are relieved from the distraction of advancing the camera all the time.
One word about flash work: Sync time with the Rollei is only 1/30 sec, but there is a 150/4 with central shutter available (quite rare but not impossible to get). Hasselblad lenses (for the 500 system at least) all have a central shutter and can sync at all times.
The screens: If found the original screen in the SL66 very dim and the microprisms which cover a large are in the center of little help. I installed a Rollei High-D screen (from the 6008, they use the same screens) and this one is great. I even find it better than the Hasselblad acute matte D. The split image finder in the acute matte D starts to black out even with f4 lenses (experienced with a 503cw+40/4 and my 555 + my 180mm), while the high-D screen´s is perfectly usable and starts to black out only with f5,6 lenses. The Hasselblad loupe has a higher magnification of 4,5x and really helps focussing, but it distorts the image in the edges. The Rollei loupe is only 3x and not so great a help, but it does not distort.
Chances are high that you will have to check and tune a SL66, since these cameras are at least 25 years old. The great thing is that the lenses will work forever, since there are no moving parts apart from the aperture, which is unproblematic. Chances are high that you need to service a Hasselblad lens, especially if you buy old. How much are you willing to spend?

Best,
Benjamin