There are few things that you should be trying to validate as part of building your business - And as per "The lean startup" (Eric Ries)... finding MVP ways to validate these early/cheaply are key to understanding how successful you will be in the future.
I think about it in this order;
1) Acquisition - Can you find and sell your idea to people (even if free trial)? Is it a good idea? (as judged by others)
2) Retention - Is your product any good? Do people want to continue using it? Did it solve their need well?
3) Revenue - If they like it, are they willing to pay? And are they willing to pay enough for you to cover costs and make a profit? What's the LTV?
4) Scaling Up - Now you know the product works and can make money... can you scale this up and over what time frame? Are your acquisition channels sufficient? Does your CPA/CAC remain stable as you scale? Can you break-even quick enough... and not run out of seed funds?
So i'd say you should definitely 'test' acquisition first... to understand viability. But you should also, quickly move on to understanding revenue. As acquisitions without revenue aren't very useful.
I guess I'm like Captain Kirk as the actor William Shatner. I don't think the answers are a clear either A or B. I would want to do the impossible: why not both?
The question would be what ways can a business model out acquisition and revenue generation in a way that can be proven to the leadership as well as to investors? It doesn't need to be something that hits a homerun. Simply a logical plan where both can happen and a logical contingency to address with that plan goes awry. I think ultimately, acquisition and revenue hold each other's hand and becomes a challenge when they are considered together.