I would not straight away use a bitless bridle on a horse that tended to take off. I agree with Figarocubed that it is largely a training issue, but then there is also the history of the horse. I have used a bosal, but that was a horse which neck reined and had a whoa, and just listened pretty well to me. I have used two cross-under bitless bridles, Dr. Cook and the Spirit bridle.(Well actually three because I made a cross under out of a modified dressage bridle before I bought a bitless, but I wouldn't recommend that approach). Both cross-unders worked pretty well on my current guy with very little training time, but he's not a bolter, and was used to direct reign on a large diameter snaffle, which I personally was not fond of. He has gotten very excited by windy days, too much alfalfa, birds or deer flying out of bushes, large scary trucks, etc. I found the cross-unders give more control that just a side-pull, and the horse can eventually feel small nuances of pressure, so it works pretty much like a snaffle (without the mouth action of course). Maybe not delicate enough for dressage unless the horsse was already trained to it. Because the action of the cross-under is around the whole head, you have to introduce it gently, don't apply too much pressure, make sure the horse is in a confined ring, and just go really gently until they get the signals. Teeth have to be in good shape, as a sharp tooth edge can receive pressure from the bridle, obviously not good for any horses attitude. If you can start her in a small area and just take it slow, make sure she does nice relaxed turns, even practice a few flexes from the ground first. But be very very light and gentle, give her some time to digest how it works. Then teach a nice relaxed one reign stop at slow speeds, walk and trot, and get that down well before you try higher speeds. So it still comes down to training, you aren't (or shouldn't) be muscling a horse into a stop with a bitless or even a bit for that matter. I found the Spirit bridle somewhat smoother in its action, but it is pricier, so you might want to try a cheaper Dr. Cook for a while if money is an issue.

 i haven't used a mechanical hackamore, but they are nothing like bitless bridles, so you can't really compare.

Hi

I would try what briddle works for you and your horse, first on the ground see what and how she reacts to, then try it in a closed in place for safety. I agree with Pepper, it is a long and tough choice and you need to do what will work best, the horse will tell you what is working if you listen.  I always give a bit (or no bit) a chance of a few days unless it is obvious that it will not work.  Then try something else if it is not working until I find what will.  I always go as light as possible, and actually prefer straight bits over broken, and no bit being my ultimate choice.  I have stbs and they race and are ridden.  To race my little horse was (and if scared) can still be a dead mouthed run away, so we wear a leather covered Italian side reining bit on him, this sounds like a lot of bit, but as we have very light hands (my husband and I) his mouth is much softer now, and for the other people he wore a snaffle and pulled a ton.  So I would also say that the bit is only part of it, as you can get pullers, or horses that have no head respect more often due to the rider than the equipment.  On my stallion we use a leather covered straight bit in harness, and he is so light he can be driven without handholds in races.  He can jog with a bitless briddle, but to train we found that while he has control unless we were to spend a lot of money on the version for driving we would not have the side to side control, but this was an equipment problem.  To ride you can ride him in anything, but usually we ride him in a simple snaffle (driving), as he has to be shown dressage one day, also often if we just want to hop on and enjoy the moment two lead lines and a halter do just fine too...

The little horse is ridden in my old kimblewick, and on occassion with just two lead lines, or my hackamore, but for the control in case he were to take off we use the bit most often, as it takes the side of a barn to stop him if he is really scared. 

So I would say go with what is best for you and your horse after a bit of trial and error, and enjoy. 

deb