I'd go with the 27" deep one - it's no problem for the hood to extend a bit beyond the counter - your rangetop most likely will as well. Even wall hoods do that - the more coverage the better.

I would call electrolux and ask them why do they have this restriction for this hood,or is it for their whole line of hoods. I think but am not sure that it has to do with the total BTU versus CFM requirements for your range and not the size. So if the 600CFM is not adequate for you range, being 42" won't work either. You are right about the size you will not need a hood bulging in you face or on top of you head when you are cooking, since you have a range top that will not go deeper than your counter tops.