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.

[Edit based on comments] This answer was based on the assumption that there is a paper publication associated with the poster. OP mentioned that there isn't in their case, but I leave the answer as it is since it might apply to other readers' case.

For many conferences in my field, being offered to present a poster instead of an oral presentation is not a sign of low quality, it's an editorial choice: the poster setting is simply considered more relevant for the work.

So before refusing, make sure that in this conference a poster is really regarded as less valuable. Even if it's the case, a poster in a good conference is often worth more than a presentation in another conference. Usually the fact that it's been accepted as poster is not visible when the paper is cited. Overall I would consider refusing a quite risky move: you know what you lose but you don't know if you're going to get anything better.

As @Erwan mentions, the value of the paper is often not affected by the format (poster or talk). This varies with each conference, so check that yourself.

This being said, I'd like to comment on the choice of poster and talk format. A talk seems more "glamorous", but has a lot of disadvantages. Unless the conference is single-track, a lot of people will miss your talk, because they'll be in a different room. Usually, everything will be slightly delayed, and you will only have time to take 2-3 questions.

On the other hand, a poster will usually be up for longer, and you can adjust how much you go into detail depending on who you're talking to. They will also have more time to think about things and ask you questions. Overall, you're more likely to get better feedback and ideas at a poster.

In short, a talk is often better if you want to get the word out, but I would actually prefer a poster if you want to discuss things with people and get input.