If you talk about the results vs performances, both are used for entirely different purpose.
With JMeter, you can speedup the communication with the server and so you can make sure that specific function works fine or not based on the response. But, it does not really gaurrentee you that your UI works as expected.
Selenium simply helps you imitates user actions on the browser of your choice. So, you can write your code once and execute it on different set of browsers to make sure that all functionality works fine on all browser.
I will give you an example here.
Hope I am clear.