They are two different things.
Tint is still a solid color.
An object with an opacity setting is transparent. You can see the crap behind it if you overlay it.
While either will work on a white background, as B said, if you use opacity you create a transparent Pantone Spot color, which Adobe HATES with a passion so large, it breaks their software. If you’ve heard of ‘dreaded white box syndrome’ or ‘visible atomic areas’ or have had all the glyphs drop out of your files on printing, had an image vanish, even had just one letter (all of them, but just one, like all of the letter “t” in a document), all of those are symptoms of having a transparent Pantone Spot color in your document. More fun, the transparent spot can be anywhere in the file. It doesn’t even have to be touching the thing that is affected.
Even better, it doesn’t make the rip choke, you don’t get an error message, it’ll happily print all those mistakes and the only way to catch them is with a proof. Sometimes you can see them if you look at your file in Overprint Preview, but not always, especially a bad font outline interaction. Gotta say that particular stunt I’ve only ever seen with free fonts, usually with bad outline files, although there was an Adobe Caslon floating around that was pretty bad. Might have been a corruption or something.
All of this still happens, BTW. It’s never been fixed by Adobe. They do not believe a spot color should be transparent.
There are workarounds and there are checks we can do, but the first check is to see if there are any spots used and if any of them are transparent.
I shoulda maybe prefaced my rant that I do wide format digital. I have no idea in conventional print what the rip will do when making plates when it encounters transparency instead of a tint. Tints it understands in line screen. Opacity? Someone with more experience there will have to answer.