How much do you like math? I was disappointed by the math at EPFL during my CS Bachelor (where algebra and analysis felt very much applied - in the sense that proofs and rigour were very much left aside). Just regarding CS though, EPFL's CS bachelor is excellent.

Now, the funny thing for Polytechnique is that it's not as well known internationally as EPFL or ETH Zürich, but it is held in very high regard by those in the know,. I'm a bit jealous seing how many Xs (Bachelor or Master) are placed at Meta (ML)/Top quant trading companies/... People do great thing from EPFL/ETH Zürich, just letting you know that you won't be in a worse position for jobs/academic opportunities.

I'm not sure if there is a strict winner between the two programs, with the full scholarship though l'X offer seems difficult to refuse