I haven't had the pleasure of being to either Greece or Spain - but I am escorting a group of high school students to Spain in March 2008. One of our destinations will be Valencia during Festival Fallas (ending on March 19) which culminates with a massive burning of dozens of paper mache effigies. This will be a high point on our trip and you may wish to consider including it in your plan.

For Greece, I would suggest staying in Athens and doing trips from there. You can easily visit nearby islands: Aegina, Hydra and Poros are very near.
Another idea would be to stay in Nafplio on the Peloponnese and take trips from there. Nafplio is a beautiful town a couple of hours drive from Athens (or train, bus or taxi ride). From Nafplio you can easily visit Epidaurus, Mycenea, Corinth and the island of Poros. You can take a daytrip from Athens to this area if you want to see the ruins but not have the hassle of transportation.
If you stay in Athens, I suggest somewhere central. The Plaka area is nice to stroll around and cuts out the need for a lot of taxis and buses.

If you decide on Spain, I would head to the south. Nerja is very pretty, as is the old part of Marbella. From there you can visit Granada and the Alhambra Palace which is beautiful.

I love both countries, and although I am visiting Spain in March for a few days, if I were you, I would head to Athens.

Carolena