Synthetic underlay is meant to ''breathe'', while the Ice & Water Shield is meant to protect areas with a high risk to building ice dams or wind-driven rain.
GAF, IKO, Certainteed, etc will have learning centers on the web to determine what is best in your situation. But with the low slope of a porch roof, I would surmise that wind-driven rain would be a serious consideration even above an unheated space with good air circulation.
It especially gets tricky were unheated space abuts heated space.
https://www.gaf.com/en-us/roofing-pr...Action=GetGrid