[fraktl]

Having "much stronger" type system cannot be anyone's definitive reason for deciding to go with Hacklang instead of PHP.

Type system doesn't guarantee that the programmer will utilize it properly, produce optimal code or even figure out the problem that is being solved by writing an algorithm.

You also need to pay attention to the ecosystem, problems that come with HHVM, the fact you're undoubtedly tying yourself with Facebook and what you're gaining / losing with the decision to go with a fork of PHP.