I have owned these both very recently. Have you? Got 10K miles of seat time in my Gen III. First time I drove my GenIV, I drove it over 900 miles home. I will stick with my statement of opinion, and I might even ride around on my horse some more. Maybe even ride my horse in the Matrix.
I will not buy another Gen III...ever. Great car, loved my Gen III, but the Gen IV is far and away a better vehicle. Quite simply, Dodge fixed everything that was wrong with the Gen III in the Gen IV. And the new Mopar PCM in it, fixed everything they effed up the first year out.
Maybe these cars are not totally different animals to you, but I am fairly certainly that MY sensibilities are keen enough to value the differences that even Dodge saw enough reason to give the generation a new number.
Having both a Gen III and Gen IV I have to agree with coloviper regarding the throttle response..wwaayyy better in Gen III. When you first start up the Gen IV and press the throttle . there is literallly a 2-3 second lag before she revs...HOPE they fixed the throttle response in the Gen V. And not everyone can get the MOPAR controller... The Gen IV is a much better improvement over the Gen III but not by leaps and bounds. Switch out the run flats on the Gen III, get rid of the crossover pipes, air intake and reprogram computers and your good to go.