Drive both. Form your own opinions ; don't take our word for it (in EITHER direction)
That being said, here's my take -
I've owned and driven bunches of each kind. In 90% of usages there's no substantial difference in performance, certainly not enough to choose one over the other if everything else is equal. Generally, Pick a truck based on overall condition of the truck.
Here's the exceptions to the above:
1) Do you put enough miles on it for fuel efficiency to play a major role? If so, then you want an 8.3. (The comparison is 9mpg to 5mpg)
2) is most of your driving at highway speed towing? If so, then you want an 8.3.
3) is most of your driving at low speed, with extremely heavy loads? If so, you want the NH250, and probably a manual transmission (M809series)
4) if you're looking to turn the existing engine up, you want the 8.3.
You mentioned turning it up. Do you have a plan to keep from killing the Allison transmission? It's pretty close to maxed out on torque rating at the stock 8.3 settings...and have you considered the likely outcomes? (No worries, I have in-frame kits for the 8.3 when you melt the pistons...)
If you need/want more than stock level power, just get one with a bad NH250. Pull the motor and put in a 400HP big cam Cummins. This is where you'll end up anyway, so just skip the intermediate steps of tearing up the others...