One is gravitational-wave modelling for LIGO-related stuff, mostly numerical code optimisation, C, Fortran, JAX, GPU/CUDA and HPC.

The other is telescope data reconstruction for AtLAST/MUSTANG-2, mainly Python/JAX, GPU computing, bayesian inference.

I like both. I mainly care about which one would leave me with better job opportunities after the MSc, especially in Europe if I’m applying normally without connections.

Are these actually quite similar career-wise. I'm pretty sure I'm over-analyzing all of this. I asked my coordinator and she told me to follow what interests me the most. I just wanted some advice please

I know it's different over there, but how much different is the question. Are you asking which would be better for continuing on to a PhD? Or is it somewhat common there to leave with a terminal masters and enter industry? I also can't imagine even if that is the case that it's better than switching to engineering and receiving a terminal masters in that.

From what you're describing industry would probably view both as a data science type role without caring specifically if you're doing GR or Astro.

GPU computing is probably going to be CUDA in either. It's like 90% of the market share. Languages are languages. Numerics will probably be similar in either. So the difference between them is only in the physics, and industry isn't going care for the most part.

With respect to your comment about the US, I would phrase it differently. If you major in physics, a PhD is not required to get a job. You can get a job with just a bachelor's degree in physics (e.g., look at the APS/AIP listings for various employers that do hire people with a bachelor's degree in physics). What is more accurate to say is that in the US, when you are looking for a job, a terminal master's in physics won't offer you much advantage (if any) over a bachelor's in physics; and as I've posted many times, a terminal master's may actually hurt you. It is true that if you're looking for a job as a lead researcher/investigator, then a PhD is typically required (with exceptions of course).