I know nothing, I am a complete beginner when it comes to formal methods. From reading about it, it seems that Isabelle/HOL is the best when it comes to automation which apparently is something you really want. It might be easier to learn (controversial, some say Lean is easier). It's been used to prove some software (including sel4 and a version of java), Apple and AWS are using it (but then I know AWS uses, or used, TLA+).

At the end of the day, I didn't want to spend more time reading about it then learning two of them (trying one and potentially switch later). The more you read about it, the more options open up (SPARK, TLA+, COQ, etc...).

I do find it ironic to read this article today given that I made that decision yesterday!

The main upside Isabelle has over other proof assistants is the existence of Sledgehammer: invoke it, and your current proof state gets handed off to a SMT solver like cvc4, veriT, z3, vampire, etc. If the SMT solver finds a solution, Isabelle then reconstructs the proof.

It's essentially a brute-force button that no other proof assistant (aside from Coq) has.