Putting aside the R vs Python question (as as noted in this thread, you can use R in a Jupyter notebook and Python in an RMarkdown notebook), I much prefer RMarkdown notebooks. RMarkdown notebooks are plain text, so you can read them easily in any text editor (which also means they play well with git, unlike Jupyter notebooks).

And it's meant to work with the RStudio IDE, so I get a much more seamless experience going between regular code and notebooks (although this is admittedly a more R-centric benefit, at least until and unless RStudio adds Python support outside of notebooks).

Of note--while Rmarkdown's python support used to be pretty bad, the most recent Rstudio uses the package reticulate behind the scenes, and it works really well!

It's easy to share data between the R and python session, and calling python from R, or R from python is straightforward.