What's worse - long meta titles or duplicate meta titles?

I have a combination of both on my site and am trying to figure out which I should take care of first - the pages with lengthy titles or the pages with duplicate titles.Thanks for your help !

Figure out which one you can fix the fastest, and work on that one first.

If you have a bunch of duplicate meta tags I would recommend just removing them until you get time to fix it. In my opinion it would be better to have no meta tags than a bunch of duplicates. Overall though, it probably doesn't really matter as Google doesn't use the meta tags to rank your pages, they only use the meta tags to help with the snippets that you see when you show up in a Google Search.

Yeah I would agree with the other guys, if one can be fixed pretty fast then sort that then deal with the other problem.

Both are seen as spammy behavior and wont be helping you this is why they warn you of it in Google webmaster tools.

Is it really a big problem with duplicate meta titles if they are well made according to SEO?

Yes, i also suggest you to remove all duplicate meta tags. Because Google considers these as Spam like keywords stuffing. Duplicate keywords means keywords stuffing or repeating same keywords many time that is totally Black hat SEO technique and Google's algorithms are against this. So remove as soon as possible.