Don't make major life decisions based on how hard an admissions test is! Both programs are hard. If you love the field, you won't mind putting in the work. If you hate your life everyday, no matter what the program is, it will be very, very hard. I mean, even if I could tell you that dentistry is 36% harder than optometry, would that mean anything to you?
If you read the comments above & SDN in general, you will find very different reasoning behind how people chose their careers. People have different values and it is important that you figure out what matters to YOU.
For example, a lot people on SDN talk about prestige and salary, and while there’s nothing wrong with caring about those things, those things just don’t particularly matter to me as long as I have some kind of stable career. However, it is INCREDIBLY important to me that I genuinely enjoy how I am spending my time, so that is what I focused on when making my decision.
For me, probably the biggest reason I chose dentistry over any other healthcare field is that it is very important to me that I spend a lot of time working with my hands and doing procedural stuff. Also, I want to have a lot of autonomy over how I practice (small business owner), I want to build long term relationships with my patients, and I definitely want to have more work/life balance than I would have in surgery.
Everyone will tell you that you should do some shadowing, and that is because it works. Just do it. It will help you assess how you will be spending your days in each career, what kind of people go into each career, what kind of opportunities will be open to you, etc. Just do it. Also, talk to as many dentists/optometrists as you can. Those question/answer sessions they hold with different professionals in the various pre-health clubs on campus are very helpful.