Suggested New Mac for Heavy R Use

Good afternoon, I am looking at buying my first Mac and thought i'd ask for advice for what I should get. I have it down to the two models below (but am open to realistic suggestions).

I will primarily be using R for machine learning packages, and the data sets are very large. If any other specs are needed let me know. 

"data sets are very large? - well, the machines listed below are certainly not suitable to run anything on large data ;) so you may want to quantify what you mean here. You want as much RAM as possible for large data since that is the single item that will cause huge drop-off in performance when exhausted and R certainly can take quite a bit of memory if this is really your only machine to run computing on. Note that in modern Apple laptops you cannot add more memory later, so this is rather important factor.

Given a choice of the two MacBook Air is not a computing machine - it?s optimized for power consumption, not speed, so the only reason to go for it is if you?re looking for a light notebook and don?t care about the computing speed as much.

Cheers,
Simon

Thanks in advance.  

13-inch MacBook Air ($1,349)
1.7GHz Dual-Core Intel Core i7, Turbo Boost up to 3.3GHz
8GB 1600MHz LPDDR3 SDRAM
128GB Flash Storage

13-inch MacBook Pro with Retina ($1,399.00)
2.4GHz Dual-core Intel Core i5, Turbo Boost up to 2.9GHz
8GB 1600MHz DDR3L SDRAM
128GB PCIe-based Flash Storage
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