The question about Marathon isn't just "is it good" but "is it better than it's half priced neighbour". As a blue nile girl myself, I'd probably say no.
The dining room is a lot nicer, with a laid back coffee house kind of vibe. The food is about the same favour wise, but in their defense you do get more variety (but the same quantity). The service is leagues better, but the food still takes quite a while to arrive (they just make a point of apologizing).
So overall, if you're cool with a dive: go to blue nile and spend $20 on a vegan meal for two, if you want a nice date ambiance then hit up Marathon, but it will cost you (a still reasonable $40 for two).