Definitely traction (marketing, sales, customer acquisition, retention) and not infrastructure.

You don't have a scalable business, you're "seeking" one. So seek it first, then worry about scaling.

Put it this way: If there's two otherwise-identical businesses, and P has "good infrastructure" with a few customers and Q has piss-poor infrastructure but great customer growth, Q will be funded instead of P every time.

The reason is that infrastructure can be solved at any time, with application of money and process. Acquiring and retaining customers is much more difficult -- more out of your control, not possible to solve with money and talent alone.

One proves you have a business, the other proves you can build systems. We're all pretty sure you can build systems, so it's not valuable to "prove" that.