This is kind of a weird topic because most top students in high school who get accepted into top 20 undergrads aren't even considering dental school as their futures. I go to an Ivy League school and as far as I know, we don't even have a pre-dental club or a pre-dental advisor. Just look around SDN, most students go to random state schools that I've never ever heard of. I have yet to meet an incoming freshman at my undergrad who was pre-dental. If you go to an Ivy or any other top 20 undergrads, you are brilliant (or legacy) but you also care about prestige and dentistry doesn't offer that like medicine, law, or business.
But to answer your question, you should go to the less competitive undergraduate school where you can get a higher GPA. While a degree from a top 20 undergrad will definitely be more impressive than a less competitive school, it's hard to say how much of a boost in your GPA the dental schools would give for going to a more prestigious school. I think a 3.5 from say Northwestern would be be equal to a 3.8 from SUNY but that would be a pure conjecture.
Another thing to consider is that college is curved, which means only the top 10~20% can receive A's. If you go to a top 20 undergrad, you will be competing against your classmates who had 3.8+ GPA and 2100+ SAT just like you. What makes you think you can beat them? Go to SUNY and you certainly have a better shot at dominating state school dumbdumb's with 3.0's and 1500 SAT's.
On the other side though, you do get a more academic environment at the top schools. Being around your peers who are so brilliant and hard working will push you to be more successful. I really enjoyed this healthy competition and going to a top school was worth it for me because it brought out the best in me. But if you think you can do well in an environment where everyone else is partying, go ahead and do it. Like someone else said, if you have a 4.0 GPA from any college and a high DAT score to back it up, you can get into any dental school.
SDN hates the idea of attending a top undergraduate institution because most of them were too dumb in high school, too poor, and went to a less competitive college. But it has its benefits. The name will carry itself for the rest of your life and it opens up a lot of doors for you. It's where the children of the rich and the powerful come. And of course beautiful too, in case of me 😍
*Waits for an angry state school undergrad to come in and talk about all the connections he's made from networking. Guess what honey, we don't have to network that's for staters.