In brief
This post is to announce the repository MathematicaVsR at GitHub that has example projects, code, and documents for comparing Mathematica with R.
My plan is to proclaim new completed MathematicavsR projects here, in this blog post, and when appropriate make separate blog posts about them.
Mission statement
The development in the MathematicaVsR at GitHub repository aims to provide a collection of relatively simple but nontrivial example projects that illustrate the use of Mathematica and R in different statistical, machine learning, scientific, and software engineering programming activities.
Each of the projects has implementations and documents made with both Mathematica and R — hopefully that would allow comparison and knowledge transfer.
Where to begin
This presentation, "Mathematica vs. R" given at the Wolfram Technology Conference 2015 is probably a good start.
As a warmup of how to do the comparison see this mindmap (which is made for Mathematica users):
Projects overview
Future projects
The future projects are listed in order of their completion time proximity — the highest in the list would be committed the soonest.

Personal banking data obfuscation

Independent Component Analysis (ICA) programming and basic applications

High Performance Computing (HPC) projects — Spark, H2O, etc.

Informal verification of time series codependency

Recommendation engines