The smaller the number the lighter use. The 400 is for the lady that messes around with it a little but doesn't really even know what she is doing. I had the heavy duty one and used to use it a lot and never a problem. My buddy in Kansas City actually has the middle one I think and he has never had a problem with his either. The heavy duty one has a 10 year (I think) guarantee while the light use one has I think a 2 year guarantee. Only about $150 difference in price I think... back when I bought mine. I think I had like 4 heads for it because 1 head gets dirty so you will need another one and so on.... then you have to rinse and reuse.
I hope this helps.