Buying used, there's really no reason not to get the 8 speed. The price is going to be the same and it's a better gearbox. New there was about a $1800 difference in price between the transmissions, all else equal.
Mechanically there is nothing wrong with the 6 speed, it's a durable and proven gearbox if you simply maintain it and the earlier problems the RFE's had (erratic "hiccup" shifts, the failing electrical connector on the outside) were entirely resolved by 2004. But in daily driving the 8 speed puts down more power and returns better fuel efficiency every day of the week.
As a matter of personal opinion if I picked up an 8 speed ram with more than ~60k miles on it I would definitely change the "lifetime" fluid in it. I know people have opinions all over the place on that topic - in the case of buying a used vehicle I find it prudent to always assume the very worst about that machine's prior life and perform catch-up maintenance accordingly.