I love both names as well and when I say them out loud, there's hardly any difference, like you noted. So, going by aesthetic alone, I'd choose
Elena. I just think it looks prettier :-)
Helena also has the probability of being pronounced 'hell-ehn-ah' or 'hell-ehn-uh' while
Elena is pretty much going to be 'el-ayn-ah/uh' everywhere, and the latter is my prefered pronunciation.