Declining an offer to present a poster instead of a paper
I don't think  you actually need a reason to decline. Saving the work for submission elsewhere is perfectly reasonable in any case. All you need to reply is "No, thank you". 

Posters are good for preliminary work and for students wanting some exposure and an opportunity to meet other researchers, of course. 

But the work is  yours and you don't need to explain why you prefer to withhold it at this time.