For an air cleaner, one important consideration that often goes overlooked is the exhaust velocity/dispersion.
Many of these units blast a narrow stream out at high enough velocities to stir up dust that has already settled, which is not helping your lungs! Settled dust is safe dust until it gets air-born.
Check to see how focused the exhaust is, the less focused, the less velocity. Also some units have much bigger ports which should reduce the focus/velocity of the airstream.
I discovered this problem after I had bought my 2 air cleaners (generally you want to be pushing the air in a circle around your entire shop, so the second unit helps ensure "global" circulation instead of the circuit only happening in a small section), so I adapted as best as I could by buying a third and running them on low when I was in the shop. When I left the shop, I would turn them on high with a 2 hour timer. Fine dust has a "float time" of about 30 minutes in still air and you should be turning over the volume of air in your shop about 6-8 times per hour.
Be sure to mount them where you can reach the controls. I think most have R/C's these days, which is worth the extra cost in many situations.
Here is a basic guide to air filtration/cleaning (but no mention of the fact you can stir up dust).
http://www.rockler.com/how-to/woodshop-air-filtration/
With that said, if your situation allows and when the weather allows (not 5 degrees outside), set up a cross flow and use a heavy duty shrouded window fan to draw air across the shop and outside (think of an attic fan for functionality). If you can do sanding and move other dust-making operations near the window fan, getting the dust completely out of the shop is always best. Same is true of dust collector; if you can set a valve to redirect to outside when weather is warm enough. Just remember, the make-up air has to come from somewhere. Your wife won't be happy if that somewhere is the chimney upstairs and you are drawing ashes and soot into the house. Lastly, if you can exhaust outside, consider where your exhaust window is located. There will be an accrual of wood dust on the plants (or whatever) outside that window (especially after rain or morning dew). A quick spray down with a garden hose will take care of it, but that is probably not something you want to add to your shop routine every time you use it. WAF matters, but if you only need to hose it down for family/friend gatherings, that is not too great of a burden.
Sorry, I may have rambled a bit too far, but wanted you to get a full picture.
