I have a PhD in Divinity from the Uk; my speciality was a theological movement throughout its history. I focused specifically on the theological aspects of that movement, its growth, development, theological implications, before then assessing its location in historic social structures and patterns. The idea that separates the two disciplines is the prinary focus. Divinty, or historical theology, focuses on the ideas of the movement through a theological lens. History tends to consider the ideas but look for a larger, more comprehensive, conception of a movement. Whilst both are very similar, with much overlap, there are key differences.

Ultimately, if deciding whether to do one or t'other, it would be best to talk through your end goal. Historians, generally, will have slightly better career opportunities outside the theological realm, whereas divinity students tend to be going down a theological route; theology is something that needs grounding in historical social institutions, but can be analysed abstractly in a way that history, generally, isnt.

I am fortunate in that I am able to teach both history and theology and, therefore, historical theology, history of theology, etc., in my university.

Hope that helped a little...