if i have to choose which is better Parthenon or Greek Islands
I happen to have a strong preference for the Parthenon. Greek Islands is usually good and reliable, but maybe a little Americanized. IMO, the Parthenon has much more interesting and authentic offerings.
If you mean roasted potatoes seasoned with lemon juice, I'm pretty sure both do that.
I've probably tried all of the Greek restaurants, in Greektown. The Parthenon is my favorite, and I've been returning there for maybe 25-years now. The sprucing-up and redecorating of restaurant makes what was always a great time even better. If you see the gal who reads the Greek coffee grounds (in her colorful costume), don't let her get away ... she's fantastic.
My two favorite places in Greektown are Athena and Venus. Athena because the food was delish and not too "Americanized", but also their outdoor patio is to die for. Makes me feel like I'm back in Athens or Thessaloniki. Venus I recommend to everyone because it's different. Rather than the typical fare of Greek food, they are more the food of the island of Cyprus, and the culture of mezes (meh-zehs). It's a different kind of food from the region and I highly recommend it.
If I had to choose between Parthenon or Greek Islands, I'd go Parthenon. Call me a snob, but I want the real deal when I go to any ethnic restaurant. "Watered down" or "fusion" is nice sometimes, but I more want to feel like I"m out of the country in someplace different with food...hence why authentic will win me over way more than "dumbed down".