I guess that "άντρας" is more modern. In fact I have tried some English for Greek lessons when I was waiting for this course, and I kept writing the ancient version άνθρωπος or άνδρας as hinted by Android (as in cellphones), and kept being prompted that "άντρας" is recommended.
I am learning on Fifty Languages and they teach "άνδρας" and that's why I wrote the comment above. I prefer "άνδρας" because of the Spanish word "androceo". :-) But anyway, I think they have the same sound (ντ and δ).
Αλέξανδρος ο Μέγας. Wow, very interesting. That's another reason I love learning languages, specially Greek.