Justice and Joy really want to learn Greek. They have a couple/few years of Latin under their belt, and they're already familiar with the Greek alphabet due to Greek Code Cracker (Classical Academic Press). We'd originally planned to add Spanish when we got to this point. There are -so- many resources and options around us for learning Spanish that it seems like a waste not to take advantage of them, and it would make it much easier for them to get a job when they're ready.
Now that we're at the junction where we could add another language, they're head over heels for ancient Greek. They want to read the ancient texts for themselves. With this much energy and excitement it would be a waste not to take advantage of it. I can easily see Justice in a field where all his ancients knowledge will be put to good use; he's a liberal arts kind of guy. And Joy would be over the moon to read the Greek mythologies in the original language. They both have time left to change their minds and go with a different language, or add a third if they're ambitious. Greek it is!
The first thing I learned while researching Greek curricula is there aren't nearly as many options as Latin. Latin courses run the gamut from really inexpensive texts picked off Amazon Marketplace and free vintage book downloads from Google Books, to courses with DVD instruction and websites with video games lined up to the chapters. In addition to the range of options, there's a wide range of levels available, from kindergarten to college. Greek can't say the same thing. (Yet?)
The Greek courses are split into three main groups, where Latin just has two pronunciation choices for you to use with pretty much any Latin book. There's the modern Greek that is spoke today, classical Greek that the ancient authors used, and Koine/Biblical Greek that primarily covers what you need to read the original New Testament.
Lost yet? My eyes spun around for awhile, like Sylvester the Cat's after a board clonked his head, and I haven't even learned the new alphabet yet! We're leaning towards Athenaze right now. It's classical, has oodles of extras on the internet and the most teacher support of the courses I've looked at. One level is actually enough to earn a high school language credit, so we'll definitely be slowing it down some. We don't have an hour+ in our day to give this. ;-)