The regular Gen IV, except for the small amount of suspension tuning done to accomodate the change from run craps to PS2s has the same suspension as the Gen III. The ACR has a different suspension. The Gen IV has a different engine, clutch, transmission and differential. The Gen IV has an electronic throttle. The Gen III has a mechanical throttle. The Gen IV has a far more complex engine ( cam within a cam, Venom programming) that cannot be modded beyond, filters, headers, exhaust and Mopar controller without spending a great deal of money. The Gen IV engine is more stressed at its stock power level. See the SRT Engineering comment on same. ( You will not be able to access this because you are not a VCA member.) There are threads on this.
I prefer the feel of the Gen III single disc clutch over the Gen IV double disc clutch but it is a matter of personal choice. I prefer the mechanical throttle to the electronic throttle. ( I have same in my Jeep and my Challenger and even after the adaptives do their thing, the electronic throttle still, on somewhat rare occasions, seems to have a mind of its own and does unpredictable things. Things that usually involve a slight delay ( think microsecond) between gas pedal push and engine reaction. But, if you do not drive the car in a very spirited manner, you will never notice this.
The Gen IV hood provides 28% better cooling efficiency.
With respect to the warranty issue, you will probably be able to find a Gen III for sale that has an extended warranty. It will not cover everything but it will cover the important stuff.
So it all comes down to what you want to do with the car. If you just want it for cruising, you can buy a Gen III. Put PS2s on it. Buy the inflator kit for flats and keep it behind the driver's seat or in the trunk. If you like the appearance of the Gen IV, all you have to do is buy a Gen IV hood and have it put on. Later, if you want more power, you can mod the Gen III engine for relatively less dollars.
Since you are not thinking of buying a 2010, if you are only going to use it for cruising, if it were me, I would buy the Gen III. If you like the experience, you can always trade and buy a used Gen IV later. And, hopefully, by the Fall of 2012, you will be able to buy a Gen V.