It seems the company is not "derisked" in any way for you or potential investors. So, there would be little justification for the lower equity stake. If the rationale is they've been working on it for many months prior, then they should have more to show than ideas. (And hopefully it isn't an indication of future pace ...)
Alternatively, consider how much you're risking earnings-wise. Assume some initial valuation ($1M?), then try to compare the estimated value of your stake to a market salary. For example, 25% of $1M = $250K, and four-year reverse vesting brings that to $62.5K/year. (If you plan to give investors preferred stock, it could be worth only 1/3 to 1/4 of that.) Hence 25% equity with no salary gives you at present moment less than a market salary. So you would need more than 25%, and it quickly becomes clear the fairest approach would be an ~equal split.
If you do join as a co-founder, make sure you are given equal access to the company documents. I would also insist on being part of the discussions with lawyers (at a minimum to verify everything is as promised). And ensure you all sign the official cap table, decide on board seats, etc.
Regarding "cash is king," as a co-founder they should be very transparent with you about their financial contributions. There are many ways to handle founder capital that don't involve depriving you of equity, e.g. loans or convertible notes. And finally if you don't trust your co-founders to be honest with you, you may not want to go into business with them.
Alternatively, consider how much you're risking earnings-wise. Assume some initial valuation ($1M?), then try to compare the estimated value of your stake to a market salary. For example, 25% of $1M = $250K, and four-year reverse vesting brings that to $62.5K/year. (If you plan to give investors preferred stock, it could be worth only 1/3 to 1/4 of that.) Hence 25% equity with no salary gives you at present moment less than a market salary. So you would need more than 25%, and it quickly becomes clear the fairest approach would be an ~equal split.
If you do join as a co-founder, make sure you are given equal access to the company documents. I would also insist on being part of the discussions with lawyers (at a minimum to verify everything is as promised). And ensure you all sign the official cap table, decide on board seats, etc.
Regarding "cash is king," as a co-founder they should be very transparent with you about their financial contributions. There are many ways to handle founder capital that don't involve depriving you of equity, e.g. loans or convertible notes. And finally if you don't trust your co-founders to be honest with you, you may not want to go into business with them.