It up to you. If you have the money, do you plan on melting the other ship to help pay for the new ship. Do you want LTI, do that matter to you.
Upgrading, you keep everything in the base package you are just changing the ship. So that all flair and insurance. If you melt it down the road you get all the money, but you can only buy back the base ship. So no good if you ever think you might melt it later and us that money to try a different ship. As you cant get it back, you have to wait until it on sale again.
If you buy a new ship, well it a new ship.
Warbond is new money only. So if you are planning on melting the ship and putting it to the new ship, you will have it without LTI (if that matters to you). You can always buy back the ship if melted down the road and you change your mind.
But it more down to where you are and what you are trying to do. It is no right answer, it more down to you and what you want out of it.