Well the 18 is about the same size as the 13.
Presume you mean 6.6 diesel since talking 2018.
You're upgrading for the "options" presumably creature comforts and newer chassis?
The L5P has more snort than the LML in your truck. You can fix that for alot less $ than a new truck.
3500 has a greater rear spring rate, rest of the truck is the same as a 2500.
Get the new truck because you want a new truck and newer amenities. It won't wildly outperform the LML, but a 50/50 bump in HP and TQ is nice.
Like lwiddis suggested, go drive it. And bring your tire pressure gauge!