Help me choose between Spring Boot/ASP.NET Core, which is better? which is faster or which is more popular? and what is the difference between these frameworks besides the language. I know C# Kotlin and Java equally well and have no problems with either language.
ASP.NET definitely, because .NET is faster than Java, Go, Python, C++ and is even 10x faster than Node.js. It beats all these famous backend frameworks like Spring Boot, Node.js, Laravel, Django, etc, by a large margin. You can see the performance benchmarks here: https://www.techempower.com/benchmarks/, ASP.NET Core is in 36th place, while Spring is in 317th place. Another one here by the .NET team: https://miro.medium.com/max/1400/1*mKgQoOIRcTZ1OMjLQ2gENA.png
Spring Boot is an opinionated framework. This is meant to say, the framework makes a number of assumptions about the User's needs and intentions and acts accordingly. The reason that Spring Boot exists and is opinionated is due to complexity creep. The Spring Framework (without the "Boot") was originally designed to simplify deployment of Java web services, but as the Framework grew in the number of capabilities and implementations, it ceased being as simple to implement. Spring Boot is the remedy, re-simplifying the whole process until so much happens automagically that it can be a little baffling. (I've never used ASP.NET, so I don't have an opinion on it. However, I didn't want to offer some insight into Spring vs. Spring Boot.)
As complementation to @Eric 's comment and as I am working with C#/.Net core for my full-time job, I can tell that:
before the .net core versions, it was painful to work with ASP.Net as it was closed to only Microsoft stack (e.g. use visual studio/windows machine, you may be tightened to use Entity framework, etc...), which was a great advantage in favor of JAVA, However after releasing the Core series, it became much easier and convenient to use .Net core for your applications as it will run it into any machine (just like java) and you are not enforced anymore to develop your application using visual studio, you can write your code on window/Linux/Mac machine or whatever you want as long as you have .Net running on it also your deployment being not coupled to windows server anymore + .Net 6 execution is pleasing fast.
I believe you should give it a shot to use .Net even in a POC application
