Ehh, just as competitive as any other field.
As for Stanford, it depends on region. Zipperstein may be a cultural historian, but Rodrigue is much more a social historian (though you better know Ladino and Turkish or Greek if you want to work with him).
The real decision to make is whether you want to be housed in a history department or a Judaic Studies program. They have different orientations and values, in the PhD training and on the job market. Either way, you'll need to be rooted in a region: Israel, Europe, North Africa, Ottoman Empire, Latin America, US, etc. No one is a Jewish historian without a region, and a good historian is going to be conversant in both settings, languages, and literatures (e.g. Leora Auslander and Maud Mandel in French Jewish history, Ken Moss and Jeffrey Veidlinger in Eastern European Jewish history, etc). Be wary of parochial programs that let you specialize in Jewish history without a solid grounding in the region's history.