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For the actual bridge vs the actual tunnel, tunnels start to outcompete once you get a length over a mile or so, so I'd guess bridge.

But since this is in the middle of a busy city, I'd guess other factors dominate, costs for land access and approach roads, river traffic, appearances etc.

The elevation needed for boats to cross under can make the bridge initial cost really high, depending on the height you need to accommodate.

Honestly, i would just trust the architects and urban planners. At least in my country, every new development i saw recently, even those i was kind of against at first (in 4 different cities) were upgrades once finished. Even when it lost something i really was attached to, i have to admit that this is overall better now.

The river at Silvertown is atleast 500m wide.

Tall and short ships and boats travel up and down it.

A bridge would cost a lot to upkeep.

It shouldn't matter. Move all cars underground, bridges are ugly (when up close / under), noisy and crime ridden.