If you are a risk taker go with option B.

If you are risk averse go with option A.

Both types are necessary. Risk Takers for trying new things and risk averse for creating stability.

My thoughts exactly.

Personally I'm a risk taker, so I'd take option B. However, there's wonderful learning opportunity with A.

Perhaps OP isn't to the point of taking the lead in their research career? I've learned much from established, tenured leadership in the past. I am now, however at a point in my own development where I make more meaningful growth towards mastery in "the drivers seat" to borrow a euphemism

Well, I'm not sure you can actually delineate here. First off, the progressing space agency wouldn't be a progressing one if it hasn't made any accomplishments (funding would dry up quick without any promising research), and it would have to have leading figures to even get any kind of initial funding in the first place, whether or not they are/have had space related careers or are the source of funding the venture/agency.

On the other hand, an already-accomplished space agency doesn't mean you can't have the potential to become one if it's leading figures, especially as it talent moves on to other ventures or retirement.

Doing space things requires non-space financial clout, and that doesn't come with a greenhorn straight out of university or college. They may have grand plans and ideas, but... they are yet not established in the community sufficient enough to command the kinds of funds required to start a space agency on their own. Therefore, even a progressing space agency has to have accomplishments under its belt with figures capable of leading the agency to its current situation.

I think the question can be phrased better.

> Would you rather work in a space agency or a space venture?

For mostly applied science but also theoretical work. Depends on the funding. I've seen great ideas never get adequate funding. Aerospike engines, Lockheed's truck sized fusion reactor,. You got a great idea but not enough money to track down all the rabbit trails.

If you can make great science on budget go private. If you to make great science slowly but surely go Govt.

I am an astronomer, and I feel that this would not be a factor in my equation. I would pick the position that gives me the most resources to accomplish interesting science. My peers do not evaluate me based on whether I am a "leading figure" in an agency. They evaluate me on my scientific accomplishment. So I want a large research grant that gives me plenty of time on a computer cluster (if I was an observer I would be asking for telescope time) and a research team of clever and motivated postdocs to conduct amazing science.

Gotta balance money with work life with passion for the work.

I also think workplace environment is important. Plenty of exploitative and toxic culture around in scientific fields, which is unhealthy in the long run, both on the individual level and in regards to projects.