In my opinion, between these 2 Spring (Java) would be a good choice. .NET is also good, but in general Spring (Java) is more flexible as a back-end implementation and it does provide a very easy REST and other integration in Spring (Spring Boot). This makes it a good independent back-end and any front-end built on any technologies like React, Angular, Vue.js can be used easily with such a back-end. WIth .Net as well this is feasible but .Net is more suitable when you are dealing with more Microsoft technologies. Also comparatively, Spring (Java) gives a better pay scale in the industry compare to .Net with similar experience. This is just my opinion based on my experience so far.