You should absolutely, positively travel first class, especially since your trip is in the peak summer tourist season. My experiences with Eurail were years ago when I was your age, but I was always really glad I was in first class. Your bride deserves a first class honeymoon, n'est pas? The $120 difference in cost is negligible when compared to the advantages of first class. There will be many twenty-somethings in first class on your trains, and few of them will be "rich."

As mentioned by another poster, be sure you have seat reservations in advance, whenever possible. I don't know how flexible your travel plans are, whether you have advance hotel reservations for your itinerary, but be mindful of summer crowds.

Enjoy your honeymoon!