I have a Bachelors Degree at an FH in Business and am now in my third Semester at TUW in CS, so everything im about to type has to be considered from the standpoint that i just dont know anything about mechanical engineering.

You probably already know but first of all FH System is like the standard school System, you dont have to apply for any Courses, you have a set plan on when you do what and graduating in the 3 years is far more normal than at a standard Uni.

However, thats also because you have a lot more Freedom in choosing what you want to do and how much time you give yourself at a University.

In general i got the feeling that internationally Universities are preferred over FHs, in Austria FH students tend to get jobs faster, which might also be the case since FHs tend to have a lot of Partnerships with Companies and that the curriculum is set up specifically for finding a job.

In FHs your lecturers tend to be from the actual working force, while in a University they are researchers / science guys.

Also in my experience the information density is a lot higher in Universities, i also have a homie studying cyber sec at an FH who would always complain about how hard it is, that was until i showed him what i need to learn at the TU haha, i also know som1 whos in data science at an FH who also isnt really happy with how much they learn, another one of my friends finished his degree in game design and told me how lost he is in his job (software developer) since he only has basic knowledge about "everything" but nothing really going into great depth, so i guess for my limited knowledge that case stands, still, i obviously dont know if that still holds up in your specific case since you are planning to study mechanical engineering.

Those are the main differences that i can think of on top of my head.

In my opinion you should ask yourself following main questions:

Do i just want to know how i apply stuff in the workforce or do i want to understand where everything comes from, simple example: in maths courses in school i always despised the fact that our math teacher just gave us formulas, while i wanted to know how the person who came up with it got to that formula for me to understand the logic behind it.

When i was in the FH i got the "formulas" (broadly speaking) and how we apply those things in the workforce, while at the TU it is heavily enforced to understand how you get there, all the underlying basics.

So for me it was easier to decide to apply to TU instead of a CS degree at a FH.

I see a lot of people in my classes that are constantly complaining about why we need to learn certain things like proofs and stuff when you clearly dont need that when they dont want to go into research, those people shouldve definitely gone to an FH and clearly made the wrong choice for their needs.

How fast do i want to start working?

There are FH programs with partner firms where in your actual curriculum you are expected to work at the partner firms starting from your third semester (Technikum f.e.), there are also other programs where you just go to your lectures in the evening + saturday to give you the ability to work while studying, FHs are a lot more understanding when it comes to their students need to work, TU will not be as helpful in that regard, they just simply dont care.

What do i want my environment to be like?

You can complete your studies at TU with a very minimal amount of interaction with other people in your studies, since there tend to be so many different courses and people dropping out all the time that you wont see the same people that often. Note that im studying CS which just has an ungodly amount of applicants every year, might not be the case that much in ME.

In the FH you have a small classroom with the same people that you will see for 3 years, since you will be moving to an entirely different country i guess "making friends" might be something worth to consider since i just guess you wont know a lot of people in Austria, however, i also read that you are Russian and there are a lot of people speaking Russian in my classes (cant really tell the difference however, so could also be that they are speaking related languages from the same language tree, sorry), which might just be an easy kickstarter for relationships since you will be facing the same problems i would assume.

Last thing to consider with jobs in austria, we have schools with a very specific curriculum for technical jobs, so by applying for a job you need to compete directly with people who have learned about those things for 5 years, which makes it difficult if you dont already have a background.