I don't think the way a potential mentor responds to an email should affect your opinion of them. I say this because I applied for PhD programs in 2011 and emailed someone from Catholic University about accepting students for the Fall of 2012. He responded weeks later and in an extremely curt manner. I thought, "What a jerk!". Well, I didn't get accepted to their PhD program then, but I did get my MA from there and that same professor was my mentor. And I loved him! Absolutely love him. Dropping a school from my list because of that would have changed the whole course of my career. Now I'm applying to work with him again as a PhD student. I don't know, I think emails are tricky. And it's not worth putting that much weight on them. You're not going to get accepted because you sent an email or didn't send an email. I mean..come on.