[strontium_90]

I prefer R because of the wide variety of statistical methods available in base R and CRAN. ggplot2 is nice, but having it in the same system as generalised additive models, robust regression, ARIMA and state-space time series modelling, GARCH packages, and a huge range of other statistical methods is very convenient.

My experience is that if there's a new statistical method developed, it will be often be implemented in R first. (This may be more true for statistical methods than machine learning or other fields).

I don't think there's quite such a wide range in the Python ecosystem yet (although it is rapidly growing, and is definitely a better choice for certain analytics situations).

None of this is to say that you necessarily lose anything other than convenience by using other languages eg Python for modelling.