The later being more likely considering the need to try and please everyone both locally and internationally. A veto is almost 99.9% out of the question since it would do irreparable damage on the international front, it would be seen as a huge foreign policy blunder by the US, the Arab league (who have stated this is the US's last chance as a ME peace broker) would forever withdraw it's support of the US in any negotiations, etc... Just consider the Iranian situation... Since pretty much every foreign policy analyst agrees with this, I'm not sure why someone would argue that a US veto is bound to occur. Regardless of which angle you approach this from, a veto would be a lose-lose proposition for the US.
Western-backed Syrian rebels say kill foreign Islamic State (Reuters)
from the article: New Palestinian peace plan may force Israel into action