Fellow Austrian here. I was in your exact boat about 6 months ago. Startup "scene" in Vienna is sad. Immigrating into the US is hard so I looked at Berlin and London.

For now I'm living in London. I considered Berlin, but I think the English practice here is invaluable. My English was already quite good when I came here, but it's improved LOADS. Also there's much more going on in general (culturally, socially), and there's more and bigger startups here (e.g. compare HN Who's hiring / startup city rankings).