Although a regular Jalapeno Pepper is pretty hot the Serrano Pepper in my opinion is much hotter than the Jalapeno.
I can eat more Jalapeno Peppers than I can Serrano peppers because of the heat difference even though I enjoy both Jalapeno Peppers and Serrano Peppers.
According to the hot pepper scales the Serrano Hot Pepper is around 10,000 – 23,000 Scoville heat units while a regular Jalapeno Pepper is between 2,500 to 8,000 Scoville Heat Units.
So by that the Serrano Pepper is much hotter than a regular Jalapeno Pepper.
But still the Serrano Pepper is not that hot compared to other Jalapenos such as the ghost pepper.