I have a tutorial website that I tried to monetize with each approach at different times.
The first thing I tried was donations. I put together something on the sidebar, and later as a footer (I felt like people would be most willing to donate at the end of an article, after finding some value). I had hundreds of thousands of visitors during the time it was up, and if I remember correctly, about ten people clicked on the donate link. None of them actually donated.
So the next thing I tried was ads. It definitely made the site uglier.
Donations is a better user experience. It feels like it comes more from the heart, and it's less obtrusive. So in the UX sense, donations are better.
But on the other hand, ads actually brought in some money. I'm not going to lie to you; it wasn't a whole lot. Even with tens of thousands of visitors and hundreds of thousands of page views a month, I was getting somewhere between $25 and $50 per month in revenue from ads. Financially speaking, that's better than nothing, but it does come at the cost of an uglier website.
Ads have become just such a normal part of the Internet these days. People are used to it. That's good and bad. On the good side, it means nobody is going to hate you for putting ads on your site. On the bad side, it means most people now mentally filter out ads, and many people have ad blocking software and won't even see it.
Had I been more of an expert in these things, I probably could have done things different to get some actual donations and/or improved my ad revenue. Perhaps even by enough to shift the balance so that donations came off looking better.
The bottom line here is that neither were super effective in the end.
Allow me to suggest an alternative choice. After failing to make ads and donations truly work, I instead chose to provide an actual product of my own, related to the content on my site. In my case, it was a book that related to a specific popular subsection of my site that contained much of the same information in book form, plus a whole lot more. While people never get rich off of books like that, the book brings in an order of magnitude more money than ads every month, it strengthens my position as an expert in that area, and helps my site visitors out in new ways. All around, it has been much better for everyone involved.
I don't think my situation is particularly unique. I think just about everybody who has a content-heavy site like this would probably do better by offering a product that compliments their content, rather than begging for donations or hanging up ads all over.