You're in a great position either way. Many people could only dream of picking between such options.

Obviously maintaining your interest level though is crucial. Could you catch up with your boss and say something like "You have a persuasive argument, but I'm really worried about getting bored. Is there something we can do that might address that?"

Realistically, with what they're spending, keeping you interested might be small-change for them. It could be something like 10-20% time to tinker on something tangentially useful to the company, or periodic breaks to get away and clear your head , or something novel at your desk.

Their offer suggests they value you and they might rather keep someone they value with 80% regular output than re-hire and get someone unproductive, with no knowledge of their systems, etc.