productivity
library
Productivity is much related to the accessible libraries for the given task. If it's all about statistical calculation, R has an obvious win thanks to its huge variety of libraries. However, when you have to manipulate/mingle data J can be easier to handle and it will become much, much easier as you get more skilled in J programming.
However, you can have both worlds using R interfaces in J.
performance
R is infamous for its poor performance. You shouldn't overuse for-loops either in J or R, though. J's got decent performance. Moreover, J code is usually terser and hence easier to change/rewrite/optimize/come up with a new algorithm. I find "coming up with a new algorithm" a big win.
community
R's community is huge compared to J's. However you have the pros and cons. Imagine the pros and cons living in a small, friendly village and in a big city.
syntax
J's syntax is surprisingly consistent compared to R's. The predictability is very high once you learned the principles.