The food at both restaurans is excellent and I actually find the value to be similar - both great choices. But the atmosphere of the two restaurants is totally different. Marco is more sparse, bright, sort of going for the simple rustic elegance route in a small room. Lucca is much larger, more white table cloth, big windows, and candles, dimly lit. Both restaurants can get a little loud although I think this is more of a problem at Lucca than Marco. I don't really think you can go wrong at either place (although I would personally never order the fried cod cheeks at Marco again). Also, Marco does a Sunday dinner that might be interesting, I haven't checked it out yet.
I have eaten at both recently and while Lucca is a very nice restaurant with really very good service, I found the food underwhelming for the price, not bad, just not terribly interesting.
Marco on the other hand is less pricey and the food is much more interesting and prepared in a less rushed style. We had great risotto which they willingly split and which was cooked just right with delicious and interesting mushrooms. The entrees are generally around 20 dollars (Lucca closer to 30) The room is a lot of fun (although when full it can get pretty noisy)and the service is good (although we had a really wonderful waitress handling a large party at Lucca who really worked hard to make our evening enjoyable despite some fooe lapses)
So to finally answer your question, I would definitely go to Marco first.