Some of that might depend on the terms of your original mortgage and how long you have left on it. Some of it may depend on when you say "still contribute to retirement accounts" if you mean that you are already maxing them out, of if you are putting at least enough to get a match, etc. and that you will be able to continue the course you are already on in that regard.
If we assume a mortgage of $250,000, and refinancing terms as you've stated, you would pay $55,379.88 on a 15-year note and $154,140.78 on a 30-year note. So, you would pay nearly $100,000 more in interest over the course of the 30-year loan (assuming you don't accelerate payments, etc.) AND that is in addition to the interest you have already paid up to this point.
One hesitation with the 30-year is that I don't know how far into the original loan you are. If you are in the first 2 or 3 years, it's not necessarily a huge deal, but if you have already paid 5-10+ years on it, then you could potentially be doing yourself a disservice by adding that time back onto a new loan, even if at a lower interest rate.
Additionally, there are no guarantees of what the markets will return over the next 15-30 years. Chances are that it will be higher than 3.5% on an annualized basis, but it may or may not be substantially higher than that. Only time will tell.
It's hard to say since we don't have a complete picture of your financial situation, but normally, when refinancing I tend to suggest refinancing to a shorter term as long as the payments work, etc., which it sounds like it does in this case. So, if you can afford the payments on the 15-year note without hurting your retirement savings, I would probably suggest going ahead and refinancing to the 15-year.
If you are really set on going with the 30-year and "investing" the difference, I would suggest "splitting" that difference and maybe putting half of the difference in savings/investments and half of the difference toward extra principal payments on the house.
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