I've flown both routes. They seem to both be difficult upgrades. Going through Houston is a more "direct" connection for most of the US including the entire west region, southeast and Midwest. To my knowledge, there are no direct flights from East Asia to EZE so you probably pick up some Asian traffic as well. Out of Newark, you have some European connections and get a the Northeast traffic from Boston, NYC and DC, which are pretty large metro areas with UA presence. With only one flight a day from both EWR and IAH, loads are usually pretty high. YMMV.