It is hard to see the difference between these two sounds (at least for a greek person) . Both are close to the Greek sound of "i" . I would dare to say that the pronounciation of ι,η,υ and also εί and οι , is closer to the i in "inn".

η, ι and υ are pronounced like [i] → ήχος, ιπποπόταμος, υποβρύχιο ι, υ, ει, οι are also pronounced in three ways: [i] → μάτι, φαρμακείο, νύχι [x΄] → βαθιά, ελάφια, αλήθεια [j] → καράβια, παιδιά, δυο To understand the symbols have a look at the International Phonetic Alphabet!