Choosing between a one page abstract in conference proceedings and full version on arXiv, or publishing full paper in conference proceedings
You are basically choosing whether you want this paper to be considered "published" in its current state, where "published" means "published in a peer reviewed venue."


If you decide that this paper should be "published" now, then you may be precluded from "publishing" it again in essentially the same form. Hence the option to "publish" only an abstract, which won't preclude later submission to another publication venue.
If you decide that this paper should not be "published" now, then for many purposes (e.g. promotion and tenure at some institutions, hiring processes that count the number of publications) it isn't credited as a publication. This may seem like a useless distinction, but bureaucratic processes are often a little bit stupid like that. Furthermore, there is a possibility that it won't ever be "published": it is accepted for publication now with 100% probability, but once you decline that offer, you no longer have 100% probability of acceptance somewhere that you'd like to publish.


Depending on the maturity of the work, the quality of this conference proceedings, and your future plans for the work, you may decide that you are ready for this paper to be "published" now in this conference proceedings, or that you're not.