The AT543NC uses the latest N-Core Class D amplifier circuit. If you have not heard Class D amps before, I would suggest seeing if you can listen to one if you are serious about it. I have heard Class D amps and they can be so extremely clean sounding. However, they lack the high frequency extension and "air" that gives music life, so to me the Class D sounds somewhat artificial. That being said, many people are not sensitive to this and just absolutely love their Class D amps. Technically, I think the AT543NC is a very good implementation of the n-core circuit as it uses fully linear power supply (many still use switching power supply on Class D amps).
Like shadorne said, the ATI4003 uses the more conventional Class AB circuit. If you don't know what Class D sounds like, I might suggest you go with this model instead.
Like shadorne said, the ATI4003 uses the more conventional Class AB circuit. If you don't know what Class D sounds like, I might suggest you go with this model instead.