A key difference here is that when you contribute to a 529 plan, the money is still yours. If your son doesn't need it for his education, you can do what you want with it. You can freely redesignate it for another relative's education (kid #2, grandchild, niece/nephew, even yourself) or withdraw it for your own non-educational purposes after paying the tax and penalty on the growth. Meanwhile just like an IRA any dividends and gains aren't taxed as long as the money remains in the account. Also it sounds like you get a tax break on your state taxes when you contribute to a 529 plan, so that's another point in its favor.
An interesting thing about the 529, supposing your kid doesn't need it, is that you can make withdrawals for room and board tax-free as long as the beneficiary is a half-time student. If you want to get that money out after your kid is done with school and you're retired, you could designate yourself as the beneficiary and sign up for a couple of classes at your local community college (French or pottery or something else you're interested in), just enough to put you at the half-time enrollment level. You can then take out the amount you spent on tuition plus some reasonable living expenses tax-free. Just an idea for a backup plan if you go the 529 route.
With a custodial (UTMA) account, the money isn't yours anymore. Once your son reaches the designated age in your state (usually 18 or 21), it's his to do with as he pleases. You have no legal right at that point to stop him from blowing it on a new car or whatever else he wants. While your son is still a minor, you have control over investment decisions, but any money you take out of the account needs to be used for his benefit, and can't be used for anything that you're legally required to pay for as a parent (food, clothing, shelter, medical care, etc.). You may expect you'll raise your son with a healthy respect for money and the wishes of his parents, but the fact of the matter is that you're taking a risk when you give money to your 18-year-old kid when he's still a baby who doesn't even have a real personality yet.
For the purpose of the FAFSA for college aid, assets belonging to the kid (such as an UTMA account) count more heavily toward the expected family contribution than assets belonging to the parent (such as a 529).
On the plus side, if you have appreciated stock you can transfer it to an UTMA and sell it under your child's name. A kid generally gets the first $2,100 of investment income per year taxed at their own rate (often 0%). Past that the income is taxed at the parent's rate. If you give stock directly, it retains its original cost basis, so this can be a way to harvest some capital gains at a lower rate. You could then take the money out for some discretionary child-related expense like summer camp or music lessons or braces, or let it compound for college.