I love both, but right now I slightly prefer
Elena because I have an
Ellen in my family and because it seems less likely to look dated. The advantages of
Helena are the less ambiguous pronunciation and A Midsummer Night's Dream.
I say HEL-en-a and EL-en-a, but I've met some Elenas called el-EH-na and el-AY-na (one was indistinguishable from
Alaina).