Spanish speakers are definitely more staunch when it come to religion. I think at the core of the issue is the average level of education of the Hispanic JW compared to the education of the English speaker. Another big factor, and possible emanates from the previous statement, is that the apostate literature, good websites about TTATT, and biblical scholarship is far more prevalent in English than it is in Spanish.
I can make a bet that within 10 years Hispanics are going to be a majority in JW land. If they are already, their proportion will increase even more, resulting mostly from English speakers defections, more than from Hispanic additions.