Do you know Latin?
If you don't I'd suggest Deutsch.

English and German belong to the Germanic subgroup of the Indo-European language family. They are analytic languages, whereas; Greek is a synthetic language. So, due to these similarities and differences, German should be easier to learn than Greek for a native English speaker.

I have the opportunity to learn through seminars either classical (ancient) Greek or German, but not both. — philosophy

Wrestling with Greek has led me to a small epiphany or two: No one ever learned any language, ever. What you learn, with suitable effort, is what you encounter in being exposed to it and can take on. The language itself always remains at a distance; one can only approach it. I'm pretty good at English, but if I encounter a word I do not know, I still have to look it up.

My fantasy had been that if I worked on and in my little playpen of beginning Greek, there would come a transitional moment when it would all fall into place and I would "know" Greek. So, first consideration: you're not going to "learn" either Greek or German through any seminars. And it's for you to figure out, that being the case, just what, exactly, you are going to learn and why. The better you can identify that, the closer you'll be to an answer to your question.

But there is also a question of draw. Both German and Greek can each exercise a powerful a attraction. Do you love the idea of one over the other? Part of the answer is that (the) language can be a gift for life. Either Greek or German, or any other. In a way like getting married, although divorce, should there be one, is easier.

From a philosophical standpoint too, both languages are significant in themselves as "houses of being" ostensibly different from English. Do you feel a preference? Finally, long term, which in any case is the case, unless you die early, you can take on both!

The best way to learn a new language is to take a lover, and I think you will find the Germans more attractive than the ancient Greeks. So take the Greek course, and the German lover.

Well, once upon a time I learned to read classical Greek, whereas German has been way beyond me. The thing is mixed up heavily with matters like emotional reactions to Germany, I reckon, partly because I find their novels-in-translation almost unreadable. Both are highly inflected languages, of course, which are tough for English-speakers: I found Chinese hugely easier. Would that not do?

I would say Greek, but I'm not entirely sure. German and English come from similar families, but that doesn't make it more easier to learn. I've heard that Norwegian is the easiest for native English speakers to learn.

German is easier to learn for a native speaker of English than Ancient Greek.

I've heard that Norwegian is the easiest for native English speakers to learn. — Swan

Right. @philosophy, forget Kant, Hegel, and the Continentals, and forget Socrates and Aristotle. Cross the corridor and attend lectures on Kierkegaard.

but that doesn't make it more easier to learn — Swan

Especially when your English is not perfect, either.