Breadth vs. depth of reading in preparation for postgraduate study
I fear answers to this question must be subjective and Am I reading enough of the scientific literature? Should I read for breadth or depth? seems to cover the general topic of breadth vs. depth pretty well. Plus you answered your own question in that you should try to do both.

As a returnee myself however I can't resist to share my experience. I also tried to prepare informally for a return to university, both deeply and broadly. I found that actually diving in deeply was very challenging since I had been out of academics and working in the software industry for 10-15 years. Consequently my math was rusty, my familiarity with notation and other formalisms was all but gone and I had a pretty huge disconnect between this new research and what little I recalled from my previous stint. If you haven't been out of the loop for as long as I was, things hopefully aren't this dour, but you can imagine this wasn't a particularly inspirational experience.

Thankfully I found catching up in broader terms made up for this. It was easier to pick up things closer to my general interest and which didn't require all that prior knowledge, but it also provided the context and motivation to understand the deeper, bleeding-edge stuff. Perhaps most interesting is that the really broad stuff, even way outside my field (physics to make math prep more fun, history and philosophy for context) have given me a perspective that isn't of direct use per se, but which adds immensely to the enjoyment of my study and personal development.

So to answer the question directly, do both starting broadly and dive in when you really want to know more deeply about a particular topic. It works wonders for motivation and they'll throw you into the deep end when you get back anyway.