There are lots of factors to take into consideration when choosing a style of hearing aid. The range of devices available is very broad from tiny devices that are completely in the ear canal to implantable devices with external and internal components like the cochlear implant. The style of devices depends on the type of hearing loss, the degree of loss, the shape of the ear, personal preference and ear health factors among others.
In general, the smaller the hearing aid, the lower the power, so people who use these tend to have low to moderate hearing loss and so may appear to be doing better because they don’t need so much help!
There are exceptions to the bigger = powerful rule as the completely in-the-canal devices can be pushed a long way down the ear canal, thereby increasing the sound pressure level at the eardrum. This means that it takes less power for these devices to get more volume. They are still not suitable for profound hearing losses. Also, some people’s ear canals are simply not large enough to accommodate this style of hearing aid or are an unusual shape which makes it hard for the device to fit in the ear - you could have a sharp bend or a very narrow canal which could make wearing an in-the-ear device difficult. Depending on the loss, aids that sit behind-the- ear can be fitted with very narrow tubing and a simple dome that sits in the ear avoiding issues with ear canal shape and size. These also allow fresh air and natural sound into the ear canal. For some people, the sensation of wearing the hearing aid in-the-ear and the subsequent effect of hollowness or loudness of their own voice is unpleasant and so prefer an open style of device. Smaller aids can also mean less functionality such as less hearing programs or no telecoil. However in these hi-tech days of remote controls, streamers and remote microphone technology, even very small devices have more and more options. The most important thing is to ensure that you discuss your individual wishes and concerns with your own audiologist to make sure that you choose something that is right for you. The audiologist can guide you and provide expert advice on clinical aspects of the fitting. Some people are happy to sacrifice a little on the functionality or sound quality to obtain the style of the hearing aid they want and that is a choice that only the hearing user themselves can make.
Second Answer: Different styles of hearing aids suit different people. The main reason why people choose the behind the ear style aid with an earmould which sits in the ear is that they are much more powerful. With the in-the-ear style, even though they can now accommodate much greater losses than in the past, you are still limited by the physical size of the aid and the way it sits in the ear canal and concha. Sometimes components such as a telecoil have to be left out of the in- the-ear aids to save space and this can reduce the functionality of the device for the user. The other reason why a behind the ear aid could be preferable is because the earmould can be made very open in what we call a skeleton style with a big air vent which lets lots of fresh air and natural sound into the ear. These are often better for people with tinnitus, good low pitch hearing or ear health issues. More recently we have seen very open devices where there is no earmould but simply a tube carrying sound from the aid to the ear and a dome to hold the tube in place. It is important to discuss all the aspects of your device decision with the Audiologist so you can make an informed decision for this important choice.