Interesting vs tractable PhD topics
You give a false dichotomy, because you should be doing a project that you are interested in and one that can be completed. There's no point in choosing a topic you hate working on for 5 years but is solvable just as there's no point in choosing a topic you're passionate about but not even those who have studied it for decades could solve (possible, but highly unlikely). Therefor, the answer to your question is to choose a solvable problem that you are interested in.
However, if you're interested in living in a binary world, then choose a project that is tractable and you hate. If you are concerned about getting a PhD, then it's better to complete a project and receive the degree than to work hard on a problem and receive no degree.