Follow heart or head: A quandary

Hi all - I'm in a bit of a quandary that I'm interested to hear thoughts.

I'm based in London and have been offered two new roles at different companies. I'm leaving my existing job because the company has lost its way and nothing is closing (I work in sales at a tech company). My background is corporate tech giant (think Google/Facebook/Amazon) and the current role is at a heavily funded start-up.

The two roles are as follows:

Company A (Heart)
- Tech startup that specialises in an industry I have a big interest in and already work in on the side. This industry may ultimately be where I end up.
- I was originally hoping to head up and build the team I applied for - but they have now changed this so somebody with more experience (circa 15 years) will run the team, and I will report to them rather than the CEO.
- The CEO is very smart and inspiring, and I feel I could learn a lot.
- The pay is a very small bump (I will actually be earning less if taking into account commission).
- They're giving me a lot of options which will be worth a lot of money IF the company sells for the sky-high unicorn valuation they are projecting (it has been running for a year and is currently valued at less than 5% of this, but it is profitable).
- The move is pretty much lateral.
- I'm not convinced the role itself has legs from the research I have done (the service is very unique and the prospective buyers I have approached have been very reluctant to show interest - and I think I might agree with them that the product won't work in its current guise).
- Culture great, people great, I would enjoy working with them.

Company B (Head)
- Advertising startup that covers an area which I don't find that interesting.
- Currently consists of the two founders, who are both incredibly smart and have already closed a pretty chunky deal (and had an offer for the company to be bought) after 4 months.
- The role would be to build out the whole commercial side of the business (sales, account management, etc). I again would learn a lot from both the people I'm working with and from the role itself (more-so than at Company A).
- The pay is low, but the equity pay-off is higher than Company A.
- The move is upward.
- Very smart guys, I would have a hand in helping to build the culture, lots of responsibility.
- I don't have as much of an interest in the industry, there will be a relatively steep learning curve, this is a high risk move.

So I'm stuck - do I go with the role at the company where I'm really interested in the industry, but not convinced by the role? Or do I go to a company where I have less interest, but a great role with a lot of responsibility?

My heart says follow the interest and forget the role, my head says follow the role and forget the interest!

Thanks in advance