Jim - there's a third type of hearing aid for people who are so profoundly deaf that the two you mentioned above don't work, and it's the BAHA... Bone Attached Hearing Aid. Part of it is quite literally surgically attached to the user's skull, and the electronic bit clips on to complete the system.

Yes, this is irrelevant to this thread, I know, but I thought I'd mention it. A member of the Morris side I play for has one, and there's information on YouTube including a very moving video of a chap who is hearing for the first time using one of these.