Either item is better than going empty-handed, but there are some far superior materials that have come out on the market during the past few years. For one thing, most reflective space blankets have a shelf-life of six months in the package before they start to deteriorate along the seams. They also don’t hold up well in the wind and won't last more than one night as the material shreds like tinsel.
I prefer using AMK HeatSheets. They're nearly the same size and cost as a space blanket but are very puncture resistant and made from a more durable material that can take some punishment in the backcountry. HeatSheets are also available in a bivy style, though I prefer the plain version as it allows for more applications (groundpad, blanket, tarp, et cetera).
A large, heavy-duty (4 to 6 mil) orange trash bag can also make a quickie shelter. I highly recommend that parents have their kids carry one (save some of those orange Halloween bags) along with a whistle as a survival-starter kit for their youngsters. Just cut out a facehole in one corner so it can be used as a hood and you have a lightweight, easily obtainable shelter that can be stowed in the pocket.
Given that hypothermia is the number one killer of people in the outdoors the world over and that most cases happen in 50-degree F weather, one should carry, at the bare minimum a trash bag. A HeatSheet is even better.