mmm, what an interesting question...
It's also what Toyota is trying to
figure out. Of course, they probably care more about the publicity in the choosing than the actual result.
So far, the most popular candidates are: priuses (of course), prii (sounds very Latin and proper), priori (apparently, the actual proper pluralization in later--also pretty cool sounding, much more cool sounding than prii [pree-ee]).
The problem is that the Latin word
prius means prior, before (like prime). In other words, it's not a noun, so it's not pluralized like a regular Latin noun. This is why priora is the actual correct Latin way to pluralize it, but people don't want to pluralize it thusly because it would be too hard to remember and most people wouldn't know what your talking about.
But I'm thinking creatively. How about prisa (pronounced pree-za)? Ya, I know that would never work, but it sounds cool.
One thing I will say is that I'm not a fan of the other two options that Toyota put out there: prien and prium. I don't think -en is even a Latin plural construction. And I think it's chintzy that they didn't include priora in the options. Didn't they do their research? This apparently is a nod to people who like words, so why not provide what would be the actual proper answer as a possibility to vote on?
If you go
here, you can actually vote on it. Even though I love the possibility of exotic answers, I'd rather keep them for poetic occasions and leave the practical answer as the official plural, so I voted for priuses. Currently, prii is leading with 40% and priuses is in second with 29%.