Everton are a bigger club and have a higher ceiling than us, especially if you believe reports about the funds being made available to the next manager.
We'll be lucky to finish top 6 next season (though it's not impossible), so if there's anytime for Koeman to cash in, it's now. Would Arsenal really want him next season if we finish top 10, however perfectly respectable that might be. Thats not saying there aren't risks with Everton. In the short-term, they arguably have a lower floor than us, so he could easily f**k up given the rebuild that's necessary there. But any manager who is confident in their ability will be thinking about the upside, not obsessing about the downside.