Which is more difficult? Getting work into peer reviewed journals, which may be a part of the thesis, or getting the thesis through the PhD and passing?

In my opinion. PhD passing. PhD is almost a one chance. If you failed it, it is hard to fix. Publishing an article in a journal can be fixed. You receive the comments, you have chance to repair your work, submit it again, and again. You have many journals. If one did not accept your work, the other might be. However, there is no one that can constitute the other value. The PhD is a degree and publishing is something else. There is no guarantee that if you published your work, then, you pass the PhD. The PhD depends on the examiners. So, you should put your mind onto passing your PhD first. That is what I think.

Mostly agree with the previous poster, although to this I would add it all depends upon the circumstances.

For example some journals will accept almost any old toot, some really won't. For example loads of people have passed PhDs but haven't got articles in for example; nature or science (and believe me, lots of people with PhDs have tried to publish with those journals). In reality if you get to viva stage at a uk institution you have a very good chance of passing, in some other countries you have a 100% chance of passing at viva stage (it is literally a formality), and in Australia you often don't even have a viva.

Like many things in Academia, much of it is really not very transparent. One thing I will say is if you have a good paper published come viva day, it's very difficult for them to fail you.

In all i'd say they are very similar, but slightly different challenges, that should be treated as a marathon, not a sprint. If you can do one, you should be able to do the other.