I'd agree, buy based on the neck: it's the one thing you can't change. If you decide you don't like the tone or trem, those can be replaced, but to replace the neck (especially to the one you like, rather than the standard Warmoth or Stew Mac profiles) will be far more difficult.
Just my two cents, but I'd much sooner take my RG520 over a Jem555. But again, that's just my opinion.