Spain or Greece?

For me it would be Spain for the food, culture and variety of landscapes and cities. For Greece it would be the people who are incredibly friendly and welcoming. Spain wins it for me.

Greece is special because of Athens and the amazing Acropolis with the Parthenon and the New Acropolis Museum. Also, great is the Archeological Museum.

Places like Delphi, Corinth and Sounion are great as well as several islands like Corfu, Mykonos, Crete and Santorini. If you go to Greece, consider a cruise for about a week to visit those islands.
Greece will be cheaper than Spain, but not by much.

Spain cannot be seen in two weeks. Don't miss Toledo near Madrid as well as Seville. Granada for a couple of days is great as well as Malaga and Rhonda. You won't have time to do Barcelona and the Basque region (San Sebastian and Bilbao), you will need another week for that.

You can take the train to get around Spain, there are several high speed trains connecting cities. It is a large country.

Recommend avoiding Southern Spain in August, due to the intense heat.

Flip a coin (I mean that in the best way). I've been to Spain four times, largely because it is an easy flight in addition to being a top destination. Greece only once, but I will for sure be back. For both, you have to be selective. Since you live in Hawaii, I would not fly all the way to Greece and then spend two weeks hopping around the islands. I would put all the time on Crete OR on the mainland (the Peloponnese plus Athens is a great trip). If Spain, pick one region to delve into, and try to avoid the most heavily visited ones (they are having a bit of a moment, and like Italy, Spain is just crammed to the gills with good places).

I think Spain would be great as it’s so easy to get around with high speed trains linking major cities. It’s also one of the cheapest places to go in Europe if that’s a consideration.

It will be very, very hot in August so I’d try to travel a bit later. That goes for Greece and Spain.

I want to emphasize the risk of extreme summer heat in southern Spain--and the weather doesn't magically becone acceptable when the calendar flips over to September. The northern coast, betwern Galicia and the Basque Country, usually has pleasant temperatures in high summer. (Note: That's because it's generally cloudy or rainy.) Much of the rest of the country--aside from the mountains, naturally--ranges from unpleasantly hot to unbearably (to many people) hot.