OP - we were in a similar position during ds’s cycle, though the full ride(s) were at MUCH lower-ranked schools. Ds received a full-ride at one school that had a Stamps, but he did not receive the Stamps there. He also received merit at a T20 - not full tuition but sizeable/significant. When he narrowed his choices, his top three were one of the full rides, the T20 offering merit, and Stanford at full-pay. Without knowing how much your “+” is over your income of $300K, we were similiarly situated to you financially, though we had about $230K saved and ds is our only. The full ride at the lower-ranked school had a program in a field that ds was strongly interested in at the time. Stanford had been and was the dream school, but my ds is pretty pragmatic. He somehow developed a spreadsheet with weighted factors for pros and cons for each of his top three choices and each school was “scored.” We used what I call the “pile approach.” The monies we had saved were ds’s “pile.” He could use that $230K as he saw fit to fund his education. If he chose full-pay at Stanford, that was going to suck up the entire pile (we were fine with covering the other $30-$35K needed over the four years he would attend). If there was money left in the pile at the end of his undergrad (because he chose one of the lower-priced options), the remaining balance was for him to use for grad school or travel or any other reasonable expenditures (save, car, toward a house purchase, etc). He knew going in that if he chose Stanford he would be on his own for graduate school should he pursue that. Med school was never on his radar.
Ds chose Stanford. There have been many allusions on here to certain career paths, post-graduate plans. Since you have not stated what those might be for your ds, I do think it makes it more difficult for folks to weigh in. Ds is now in MBB consulting, and I agree with others that for certain (narrow) fields where one attends undergrad matters. I also agree with the poster that attending one of those undergrad schools certainly does NOT guarantee a pathway into one of those fields. The competition is strong, and the number of actual hires is few, at least for MBB consulting (not sure about banking). The opportunity is there for his firm to cover tuition plus some living expenses for an MBA under certain circumstances. They won’t cover it just anywhere. I am not sure of all the ins and outs of how it works, but that is now his circus, not mine.
I never understand the “he can go there for grad school” statements. That is not guaranteed. Of course, without knowing your ds’s plans, whether or not grad school needs to be factored in is an unknown.
Your ds is clearly an intelligent young man. I understand wanting to guide him and get all the perspectives that you can. But, IMO, it is ultimately his decision. Tell him his “pile” amount and stop talking about it to him. Give him some space to ponder. He still has a month to decide. He needs to own whatever he decides. Revisit (or visit) both schools if possible. It took my ds about a week to reach his conclusion, and it was really hard for me not to talk about it/ask what he was thinking over and over again. If your ds wants your views, he’ll ask. If the pile for Stanford doesn’t work because of little sister, then take it off the table.
As someone who is an only child and who has an only child I am probably not the best to weigh in on equity among siblings. Providing BOTH of your children with a debt-free undergraduate degree seems “fair,” to me. You have indicated that little sister is likely not going to be attending the same caliber of university as her brother. Level of elitism and level of price are not, however, correlated. Only you can know how much you can save for little sister over the next four years. Only you can know the family dynamics of fairness and equity and what that means in your household. To me, making sure the dollar amount is even is irrelevant, but obviously others differ and believe each “pile” should be exactly the same.
All the best to your ds as he decides. Please let us know the outcome.