That is a hard question for me. Greek has a very familiar alphabet, and the syntactical language seems a little more English-like. Hebrew, as others have said, is very foreign. The alphabet looks other-worldly (although it is very fun to read, even if you can't translate anything!). However, after learning Hebrew, I felt that it was simpler, for some reason or another. Right now, I think I feel a little more comfortable in Hebrew than in Greek. The hardest thing about Hebrew is weak verbs. Some of the ways certain verbs conjugate is absolutely wild. Sometimes, if a verb is doubly-weak (i.e., it has to weak consonants), only one root letter of the original three can be found, yet the word can be five or six letters long! Therefore, you have to be very familiar with prefixes and suffixes in order to be able to determine what the lexical form actually is.
In the end, I would say that I found them both equally challenging in their own ways. However, I actually found that the foreign nature of Hebrew actually facilitated my learning of it, since I went in with more of a blank slate than I did with Greek; I had no preconceived notions.