What, if any, are the quantifiable differences between a rhectoric/communication PhD from an English vs a Communcation department? I'm currently pursuing a master's in Communication and plan to seek admission to a PhD program afterword, most likely in the field of technical communication. Many schools offer such a program fully or partially online, but regardless of how the program is offered, many (nearly all, in fact) of them are offered through the institution's English department. I've been told by several professors that communication programs in English departments are inherently inferior to those in Communication departments. Is this in any way correct, and, if so, why/how?
Yes. These two departments are very  different, and social scientists (and business employers)  will certainly think the English department version is inferior. I'm not in either field, but when I look at the research done by faculty in the Communication department on my campus, very little of it would be done by anyone associated with an English department. Communication departments study topics like implications of new Internet-based communication methods (Facebook, etc.) on stability of authoritarian regimes. AFAIK English departments would not be interested in such topics. 

When looking at any department for a PhD, spend a fair amount of time looking at publications from its faculty. 
The fact that you  might be interested in an online PhD suggests you are not planning on a conventional academic career, but rather want to pick up specific skills. What skills will you learn that have value in the job market? For technical communication, many employers will be looking for  people familiar with IT and digital media of all kinds. Words are often only a  portion of the communication content, so you need a department that looks way beyond text. 

It's possible that there are some Communication programs inside English departments that are not at all like conventional English departments.  Perhaps I am biased - but most of the people  interviewing you for jobs after graduation will have similar biases. 

I welcome contrary responses.