Professor startup vs FT at trading firm
tl;dr: I've been offered a position to join a technical startup by my advisor for 3% equity (with no salary until funding), but I've already accepted a FT position as a research engineer w/ above average new-grad pay at a big trading firm (e.g. Jane Street, DE Shaw, 2 Sigma).
I recently finished my undergrad and accepted a FT offer as a research engineer at a big trading firm. During my last few months of my undergrad I've been committing a lot of time researching part-time with my advisor and plan to submit a paper soon. In the meanwhile my advisor and a couple other professors have been building a company out of the research they have produced. Before accepting my FT offer I was asked to join the company since I've more than proven myself (I've basically committed full-time to working on my research with them). At the time the company was still very early on and I ultimately decided to accept my FT offer.
I was recently pitched to again by my advisor that progress has been looking great and they have been getting decent attention through their POCs. They told me they would like me to join the company and I would be getting 3% equity (vested over 4 years, not fully diluted) as a founding engineer, the same as the other engineers (all students of the professors) who have joined the company. They said they were looking for either a quick acquisition within 2 years or at least a decent early series funding round in the next year and that my upside could potentially be equivalent to or more than my FT offer. My advisor also did strongly emphasize the chance of success is still very low but the upside could be huge. I do believe he has my best interests in mind but I can also see where he could be biased in his intentions. I believe the problem space they are working in is very niche in a good way and could have huge potential for a decent exit.
One of the professors have had experience with starting a company and getting acquired early by a large tech company early so I believe the team has the experience and connections to pull it off. My advisor told me that this is a great opportunity considering that since I just finished school I have no significant commitments like a family to worry about and that this would be a much better experience being part of the core team building a startup than taking a FT job and being another cog in a big machine. He said I will even be able to hedge the risk by pursuing a PhD at the same time since the research areas overlap.
My main concerns are:
1. I will be receiving no salary but only a promise of some equity until funding is received (or when the company exits and I receive my share + an offer @ acquiring company). I should be fine financially since I will be able to receive some grant money for being a PhD student at my school, but not having a salary is obviously less optimal.
2. I will have to burn a bridge reneging on the offer.
3. Nothing is in writing yet until after I commit fully and renege on my FT offer. That being said I do trust my advisor is not intentionally trying to cheat me since he'd like me to do a PhD with him afterwards regardless of the outcome.
Pros I can see:
1. I plan to pursue a PhD after a few years in the industry, so being able to hedge a PhD with this startup is very optimal. This will also allow me to finish a PhD earlier than planned and thus my starting salary if I do end up going into the industry will begin higher as well.
2. I've always planned to start or be part of the founding team of a technical startup. Being offered this opportunity as a student with few commitments seems like an opportunity too good to pass up, although with huge risk given the concerns (see above) I have.
I am leaning to not renege on the offer and continue with my original plan of pursuing a PhD after a few years of working FT. My advisor did say if I do decide to come back after a year I will definitely not be getting anything close to the equity package being offered currently.
It seems that this is a huge risk to be giving up a good FT offer for a moonshot, but I'd hate to look back and see that I passed up on a huge opportunity to catapult my career. Apologies for the rambling.