I recommend Zero over the first game in the Agarest series -- mostly because it has less boring random battles, it is harder but more rewarding, and has a digest mode with the full storyline of the first game. So it is better all around.
The gimmick in these games is that you play successive generations of characters which you yourself can create by choosing a spouse. Different spouses will lead to different children who will be good at different things. In Zero specifically, you also create your main character at the start and can customize his every detail, making for a nice number of options.
All in all the Agarest series is nice, if a bit clichèd, set of strategy RPGs. And the Steam versions patched some bugs and put in a few nice touches that make end-game grinding much more bearable.