Which is the least stable trihalide of nitrogen?

Which is the least stable trihalide of nitrogen and why?  

I have two conflicting theories, one is that fluorine-fluorine electron repulsion will make $\ce{NF3}$ least stable and another is steric reasons of iodine which give $\ce{NI3}$ as the answer. 


Which is correct?




[Nitrogen triiodide $\ce{NI3}$](http://en.wikipedia.org/wiki/Nitrogen_triiodide) is an extremely sensitive explosive that explodes with the slightest touch when dry. 


[Nitrogen trifluoride $\ce{NF3}$](http://en.wikipedia.org/wiki/Nitrogen_trifluoride) is so stable that it is a greenhouse gas global warming concern, with lifetime in the atmosphere of hundreds of years.


$\ce{NI3}$ is unstable due to steric strain as you are saying.


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