When We Collided and All The Bright Places both follow characters suffering from bipolar disorder so I thought it could be quite interesting to compare the two of them because there are a lot of similar aspects and a lot of fairly different aspects. I do warn you, there will be spoilerish content so please don't read on if you don't want to be spoiled.
So to begin with, one similarity was that they both had love interests and the love interests were both grieving at the time they met Finch/Vivi. The books both seemed so have a focus on how both Vivi and Finch brightened and changed the lives of grieving Jonah and Violet. It was very fluffy and lovely until you noticed how odd Vivi and Finch were acting.
It wasn't revealed until the end of both books that the characters were suffering from bipolar. Although, in When We Collided, Vivi has been diagnosed prior and Finch had been professionally diagnosed yet. So this is an interesting point, Vivi and her mum had knowledge of what Vivi had and knew how to maintain it but Finch and his family didn't have a clue really.
Vivi and Finch both had very manic periods towards the middle/end of the books and they were both kind of very similar when that was happening. It was horrible to see them both deteriorate like that and yet, I couldn't stop reading because I was hoping so desperately that they'd both be okay. It was also, really interesting to read, if not painful.
A couple differences would have to be for one, the endings but that's a huge spoiler so we won't go into that. Also, Vivi's mum was a lot more involved whereas Finch's parents were very uninvolved which is why I assume, he never got professional help. Vivi was having a relapse due to not taking her medication whereas Finch wasn't on any medication to begin with seeing as he was never officially diagnosed although it's questionable to as wether this is the first time Finch had been manic like that. He suggests he went into a sleep quite often but I'm unclear on what that kind of means of the bipolar front.
So, that concludes my comparison. I think they're both very interesting and insightful books. I couldn't pick a favourite, they're too similar yet different at the same time. If you're interested in reading more of my posts about When We Collided then you can check out my review or my soundtrack or even my letter to Vivi.