Which runs hotter, MacBook Pro or MacBook Air?
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.