I'll preface this by saying I haven't ever done work on the Microsoft stack, so take this with a grain of salt.
Community support: Java tends to have a more active community (and much more diverse universe of new open source projects) than .Net (related to below).
Difficulty level: I'd say the Java/Spring stack is more difficult because Java developers tend to use more community projects and have to know which ones to choose in which circumstances (e.g. what logging framework to use? What database connection pooling library to use? What testing framework to use). That being said, most decisions you have to make have choices that are vastly more popular than others. My impression is that .NET guys use stuff from the standard library almost exclusively. If it's not provided there, it's a problem they never thought to solve or maybe write their own (usually the former).
Career prospects: I'll get hate for this probably, but Java/Spring has more jobs than .Net. .NET Core even more so because it's new. Most .NET jobs out there are going to be not Core, and not on anything other than Windows. LinkedIn search for jobs in United States shows 97,103 for Java and 36,448 for C#. That's not to say some individual city might not have more .NET than Java, but by and large Java is bigger. Also, .NET is not dying at the same pace as say Ruby on Rails (sorry Rails fans). I'd say it's ticking very slowly lower, maybe even holding steady. I wouldn't say you're screwed career-wise if you choose .NET. C# is also pretty similar to Java from what I've seen, and I know professionals that have transitioned to Java (though interestingly, I don't know any that did the reverse). Several companies have basically no .NET footprint, for example, Amazon, Google, Facebook, LinkedIn, Twitter all have Java, but basically no C#.
Specialized fields: Neither Spring or .Net are going to be a thing in a field like data science. That's exclusively Python (some R) for the most part. The only exception being that there is some Java in Hadoop, and Scala in Spark (which runs on the JVM). But I think those are generally more for data products being created, and not data science work. I'm also under the impression this is increasingly less the case than historically. Some other specialties might make Java useful, for example Android development.
In short, Java is the COBOL of the modern enterprise (which is both a good and bad thing). I recommend it to anyone over .NET, but not for technical reasons. It's for reasons related to the questions you asked. There are actually reasons I think CLR and C# are actually better from a technical perspective than Java (unsurprisingly, since they had the benefit of hindsight). But that's not what you were asking about...
Hearing you say " Java is the COBOL of the modern enterprise" sounds kind of depressing....
What I mean by that is that it's ubiquitous and stable. And that I'm maybe a touch cynical after being a Java guy for over a decade.
Innovation has gotten better under Oracle. Sun kinda let the language languish from a lack of new features (e.g. functional programming). But the last several years have picked up and I think has made companies that were looking at Scala or whatever alternative languages decide that the updated Java was good enough and they no longer needed an alternative.
Let's be honest though, all these languages (Python, C#, Java) aren't exciting. They've been around a really long time and are stable workhorses. But that's not a bad thing. A language that has a lot of exciting new features is just gonna break my shit more likely than not.
I'd also add that Python is chipping away at Java's dominance it seems (although it's not happening in the city I live in). So good on you for having that in your toolbelt.
Python for sure has been dominating for the several years. However, I've heard that when it comes to the field of data engineering, Java has more edge. Perhaps a lot of tools like Apache Spark and Kafka are based on it and it also the ability to handle large scale data, but I am not sure about it. Another reason of me asking for Spring/ASP.NET Core is the nature of Java and C#. Knowing a statically typed language (and the relevant frameworks) forces you
to think about your code in more cautious manner. They give a different perspective about writing code. But again, these are just my thoughts.
That is true. Java/Scala are pretty big in data intensive applications, though this is a fairly highly specialized area. Since most data science is done in Python, I think fairly often folks just run their stuff in Python and even though it's not all that performant, it's often good enough. Though there's definitely overlap in that problem-space.
It is useful to understand how both dynamic and statically typed languages work. But I wouldn't say a statically typed language makes you have to be more cautious, I'd say it eliminates a category of problems (things not being the type you expect at runtime -- you always know exactly what types you're dealing with). Java is rather verbose in its syntax, but that's not strictly related to it's lack of dynamic types. Scala is much less verbose, but also lacks dynamic types (but it does have type inference, which is one of the things that reduces the amount of ceremony). Static typing also have better tooling (it's easier for an IDE to autocomplete stuff, etc), and better performance (in theory, someone probably could create a dynamically typed language that performs as well as statically typed languages, but it has never happened yet).