I would quickly take a look at some of the methods for recent publications in the MRI area and see what some of the top research groups are using. My personal preference would be python due to the deep learning benefits that Mehran has mentioned.
It also looks like there are far more open source resources for Python than R based on the following GitHub search results -
"language:Python MRI" - 1868 results
"language:R MRI" - 196 results
Although this is no guarantee of the quality of the code, it seems like python is the more popular option.