I use a Hamilton Beach 200 Sqft True Air model, after an allergist found I have serious allergies to dust. He recommended I just have a simple model like this for my bedroom. My asthma is still acting, but a combination of this air purifier, singulair, and dust proofing my bed have made my allergies nearly non existant.
So I am guessing the air purifier did some good. It's amazing how much dust gets stuck in the filters, meaning that's how much dust that was circulating around in your room.
This particular Hamilton B. model I use has been very good. Any decent air purifier seems to help and work for most people, the biggest complaint I've heard of is their durability and their noise. I've had this model for about 2 months with no problems yet.
It is loud if you have it a full blast, but I usually have it at its quietest setting, and it just lets out a dull, white noise, that is not disturbing at all. And I am a VERY sensitive sleeper, so I am sure you would be alright with the noise level.
A HEPA air purifier is recommended. The Ionic ones are not - they actually emit some ozone, which is harmful to your lungs. The EPA had a report on this for a few years, and they are just finally stating this in Consumer Reports. So definately go for a HEPA filter. I use one at work, but since there is a mold situation there, I still have a major problem with my breathing. So, I am not sure how much it helps, but it could be that I would be in the hospital if not for my air purifier, which I keep running on high every day while I'm there.