thats not a bad idea... i wouldnt own a diesel if i didnt have a econo car as a DD. i dont have any issues with the vibration problems.. you also have to remember that the people complaining on a forum about specific problems is only a very small percentage of the people who own those vehicles. i have not heard of any 14's with the issue. only 13's down. also, obviously make sure you bring the truck up to 60-80 on the test drive to be sure it dosent have it.
the only difference is leaf springs... no reason to get the 3500 with a stiffer ride unless your actually towing 10k plus.
you should be able to get a killer deal on a loaded up 14 when the 15's come out... just like the 14' 1500's, you wont be able to get even an OK deal for a long time..