Question: I currently use a 13" Mid 2011 MacBook Air, and the thing I dislike most about it is that it can get very hot. I'm particularly physically sensitive to the heat for some reason, and at its hottest it makes me uncomfortable even to just sit at a desk with my hands on it.

Anyway, I'm in the market for a new laptop, and I'm considering the current 13" Retina MacBook Pro and 13" MacBook Air. I'm wondering: Which one runs hotter? (Under light use? Under heavy use? At max heat?)

Answer: 
This question seems a bit reversed. All Apple products are designed to let the CPU hit 90°C before the CPU is throttled. So all of them, when given an unlimited workload hit the same design temperature. 

What does change is the workload required to saturate the CPU of a MacBook is lower than what will saturate a MacBook Pro. You could just go on the wattage of the CPU to pick a model that doesn't put out so much heat if that's really your main concern. I find most people buy today based on screen size, port counts since ram and CPU and storage are the same for just about all the portable computers. 

Alternatively, adapting to use an iPad Pro would be a good choice if you really want efficiency and limited heat buildup. 

Here are some references for TDP - thermal design power - that translates directly into how hot a processor will run. For MacBook - this correlates pretty directly to how hot the case, machine runs and also how much power it takes to run (battery drain times).

* <http://ark.intel.com/products/88199/Intel-Core-m7-6Y75-Processor-4M-Cache-up-to-3_10-GHz>
* <https://www.apple.com/macbook/specs/>
* <https://en.wikipedia.org/wiki/List_of_CPU_power_dissipation_figures>
* <http://www.apple.com/macbook-pro/specs/>
* <http://ark.intel.com/products/97122/Intel-Core-i7-7700T-Processor-8M-Cache-up-to-3_80-GHz>

MacBook is 4 W (burst to 8W) for the most power hungry MacBook in early 2017.  

MacBook Pro is 35 W (burst probably another 15 W for GPU) for the most power hungry MacBook Pro 15" with GPU in early 2017.

2016 and older models will run many more watts so check carefully if you aren't buying models that are current in March 2017.
It all depends on your usage (the load).

For normal uses (the CPU newer runs at 100% for long times) the MacBook Air would be the colder solution.

The MacBook Pro is the more power hungry machine (retina display) and could run hotter.

One way to see this is in the Battery life. MacBook Air can go up to 10 hours on a single load.

Your MBA might need a SMC reset to make sure the Fan control is working correctly.