I am a huge fan of Banking. Like all things though, “Better” is a matter of situation.
Terraforming is better in the long term, but Banking is more versatile and better when you are at war.
Terraforming is good, too, but it is indiscriminate. banking gives you funds do focus upgrades on key planets. For example, I dont need/want most of my planets to be upgraded near my borders, especially on economy. The advantage that Terraforming gives for those planets is largely a sunk cost for me until such time as those planets are no longer border planets. banking aloows me to target which plaents get what upgrades more effectively. Granted, in the very long term, terraforming helps with that too by lowering costs (slowly) but 1) thats really hard to see and 2) it doesnt giver you the ability to surprise unwary opponents with drastic upgrades in the same way banking does.
Banking gives you cash in one lump sum – which is never, ever a bad thing. That regular infusion of cash allows you the freedom to do things outside of planet upgrades, include bribing opponents or trading for tech. Indeed, trading for a banking upgrade is ALWAYS a good idea. if you can trade it for close to the transfer cost, the upgrade pays for itself in no more than two cycles. .
The other hidden advantage of Banking is that it is immune to predation by your opponents. Economy can be degraded, as can industry and science – but banking is all yours. In a defensive battle, or a battle where you do not have the upper hand, Banking is invaluable because it provides a steady and reliable stream of income your opponent can’t touch.
They really are two sides of a very similar coin. Terraforming saves you a lot of little chunks of money that quickly add up. Banking gives you a big lump sum to do as you please.
But in an arm’s race? Or in a nasty fight? I want my Banking to be robust. It gives me large funds I can use on strategic planets and in such a way my opponent cant take those funds away. Thats a big deal.