It's tricky to say, because the historical context in which literature existed is radically different. What we see as 'classic literature' is the cream of the crop that has survived the ages, from a historical situation in which, due to expenses, literacy rates, and general standards of living, literature was not as viable a business as it is now.
Whereas by contrast, during the 20th century and beyond, a far greater volume of books were published - meaning that proportionally, you will get more dross, but you shouldn't let that colour your view of what the quality of the good stuff is like.
However, one thing that I can say, generally speaking, is that stylistically, structurally, modern writers have improved upon and refined their language, dialogue, and story structures, having had the opportunity to learn from both the successes and mistakes of the classics.