From z/OS to AS/400? Career question

I have an offer lined up that pays substantially more. I have a few years as a COBOL dev in banking but mainly in z/OS. I could probably find a higher paying job doing what I do already but now I have an option to jump into AS/400 instead.

I’m wondering about advice or just general things to consider. What would the long term path be? I’m not as old as some of the other developers and just want to have a future 10-20 yrs from now still.

Thanks in advance.