Is it better to have a strong thesis, or to follow personal interests during PhD? I'm a second year US PhD student in applied math. For the last year I've been working on one project with my advisor and another one of her students. We have also started a collaboration with a professor from another university on an unrelated topic. Both of these project are progressing at a fairly expected rate.

Recently, I have made headway on a "personal" project, which I expect to publish as a single author paper. As a result of this, I've been in contact with some people I met at a conference, and am planning to work on the follow up analysis with them and my advisor. I think this paper and the subsequent analysis will be significant enough that it could serve as the bulk of my thesis. However, I'm not that interested in this topic, and definitely don't want to spend my career on this problem.

I also just reached out to a new professor in our department about some projects which could be much more aligned with my personal interests. 

At this point my main goal is to get a good postdoc and then a good tenure track position. I'm fine with grinding through any work for the next few years if it will help me get there, so my question is whether it will be more beneficial to continue with the personal project on which I've made the most progress, and end up with a very focused and "citeable" thesis, or to pursue problems I find interesting, and end up with a broader but perhaps less impactful thesis. 

In either case I expect to work fairly closely with my advisor and one early career academic from outside my university, but if I pursue my personal interests, I'd probably end up working closely with at least one additional person.
The papers are more important.   They have way more impact than thesis.  Especially on technical fields.  

Your thesis will be pass fail usually.  May be written up just because advisor says you are ready to go.  This us different in humanities.  

Depending on your results you may even have to be creative and mash a snowball together from scattered work.  Assigning a theme even if there isn't quite one.   Not ideal.  But happens. 

It can also happen if you are strongly productive to just pick one part of your work time be the thesis.  It is also not uncommon to see students do a couple areas and eventually abandon one usually the first.  Often because it has more results.  This can even happen quite late in grad school.  

The dirty secret is that the thesis is just a school drill.  But papers for the literature?  That is playing with the big boys.  

A different question to ask is if you will produce more by sticking to one area or moving around.  Really you need to make your own guess on efficiency and how target rich the areas are.  In all cases you need to consider what to invest time on. But I don't think of it as thesis versus papers.  It's always papers.  Just on what topics.  Thesis is just a glorified term paper.