I've read some studies that claim the human ear cannot notice THD below as high as 3 or 4%. I seriously doubt you could tell below 1%. When you get as low as 0.1% the number becomes absolutely meaningless.
The 0.02% difference between the two amplifiers matters as much for sound quality as the look of the remote control. Your speakers will be the weak link as far as distortion goes, not your receiver. A few years back, the manufacturers really started to boast about THD numbers, before that most people I know rarely considered it when comparing two amplifiers.

Don't worry about the 2400's sound quality...it's got plenty of reserve power and sounds great...I have the 1400 model, it replaced an older 795a model. Same thing as the 2400 basically, and I was quite surprised at how much better Yamaha's newer receivers sound. I'm sure you'll be impressed with it.

I use a couple of Adcom power amps to run 4 of my 7 speakers right now, as my room is quite large (26 X 22) and the extra power helps, but in most setups I imagine the receiver would do fine on its own. If you're going to buy some separate power amps, the THD becomes even less meaningless.