Sop focusing on those two cabinets and look at the bigger picture of the overall storage in the kitchen as a whole. And at WHAT you intend those two cabinets to store. Most people would want 1 of them for spices, and it would be adequate for that with a 15''. Post your overall layout, and start thinking through in your head what goes where. If you have a pantry, it's probably a no brainer that a 42'' will work better. If those are the only wll cabiets in the whole spce, maybe the 18'' would be more functional.

hollysprings makes a good point that considering the bigger picture is important. Once you know what you will likely put there, it'll be easier to decide whether it would fit in a 15" cabinet vs an 18" cabinet.

With that said I have a 36" vent fan over a 36" induction cooktop, because it made sense to me, and I find it works quite well. I previously had an OTR microwave with an anemic recirculating fan, and my current fan is just worlds better than that was. While a 42" might work better, the only times when my fan isn't doing the job is when a steaming pot on high is jetting steam horizontally due to an ill-fitting lid, and in these cases I don't think that 3 more inches in width would make that much of a difference.

Ditto much of what Bob just said. We have a 30" over a 30". Not perfect but made sense in the whole scheme of things.

I know for sure I want spices in one of the cabs -- and the spice organizer I have and like is just under 15" -- which if I remember correctly will not fit in a 15" cab (side walls take up space :-) )

Not sure what will go in the other cabinet...

Thanks for your input -- I'll need to think on this for a bit.

My goodness, a 36 inch hood with adequate cfm will be just fine.