I cannot but agree with the above response!
I don't know, it just doesn't quite make sense to me: knowing that a PhD means you'll spend a minimum of three years working on one topic, should it not be the topic that determines whether you'll apply for a PhD at all?
I'm a prospective PhD student (in English) and it's definitely because of my topic that I've started to apply here and there. I am actually so into it that I've decided that, PhD or not, I'll write that thesis, lol. It's not like I can dream, as I'm not a native speaker, to get a job in academia with it anyway. The PhD comes in only because I fantasize about being able to do this in 3-4 years full-time instead of 10 or 15 part-time.
I'm afraid if you go the other way round you may well be signing up for a few years of hell... I can't swear as I'm not into the thing yet, but it seems logical