It's extremely difficult not to be biased. Usually the people that answer these types of questions would be Mets or Yankees fans, other fans usually don't care. Keyword: Usually.
As a Mets Fan, I have to go with Citi Field, but like all of the other stadiums it has its flaws.
Cleanliness: Pro (Well it's a year old, it can't be that dirty.)
Food: Pro (There is a large variety of food. From pizza, to burgers, to sushi (yes, sushi), to knishes, you get what you want.) (Some of the foods are limited to specific seats)
Seating: Major Pro (Every seat is made to have a perfect view of the field. They are all angled in the direction of the center of the field)
Resemblance of Team: Major Con (If you went into this stadium, you would think it was used by either the Brooklyn Dodgers or the New York Giants. When you walk in you see the number "42" in blue for Jackie Robinson, the shape is of Ebbets Field, the late home of the New York Giants. Fans complained, which finally led to the addition of "more Mets".
The Yankees on the other hand:
Cleanliness: Worse than the Mets, by extremely little. (You have to infer this. Since the capacity of the stadium is higher, the dirtiness, is higher.)
Food: Equal to the Mets (You got your Hebrew National Hot Dogs, and your Zeppelini, not to mention your Southern BBQ)
Seating: Little Bit Worse (Seating is more cluttered)
Resemblance of Yankees: Major, Major Pro (It is all Yankees, You got everything Yankees. The statues, the engravings, the atmosphere. If you want the Yanks, you got em'.)
Overall: I prefer Citi Field, because your question asked about looks. The Yankee Stadium and Citi Field, are in my opinion the two nicest stadiums. (Pretty Biased, Sorry).
I choose Citi Field because, the Yankee Stadium looks exactly the same, the same shape, the same atmosphere. After 75 years+ in existance, I would expect a change, all I feel from the New Yankee Stadium, is more of the same.
Citi Field, is nothing like Shea, it's clean, the ground doesn't shake when there is a homerun and it's different from most ballparks.