Which factors encouraged more immigrants to settle in Northern states than Southern states during the mid-nineteenth century?

The primary factor in this situation was economic. Rapid industrialization in the North created manufacturing centers throughout New England and the Northern colonies. This, in turn, provided more employment opportunities than that of the South. The Southern colonies were much more agrarian and relied on slave labor during pre-Civil War immigration. Upon landing in America, immigrants were desperate for a job and extremely poor. Most took the first jobs they could find and those were often times near their ports of entry.
The Northern ports also served as a logistical reason for their settlement in the North. Nearly 70% of all immigrants traveled through the New York port in the 19th century. Imagine yourself as an immigrant, traveling to a new country with essentially no belongings. You'd likely try to find a job quickly, which meant not traveling too far inland. As more immigrants settled in towns across the North, word spread back home of the promise of America. This brought on waves of new immigrants who settled near those of similar culture who had already made the trip.

The North was considerably more industrialized than the South. Its economy was also more advanced, more modern, better financed, and could draw upon substantial reserves of capital for business investment. The Southern economy was based primarily on agriculture, most especially cotton. Slave labor formed a major part of the workforce, so there were relatively few opportunities available for immigrants to come and work in the Southern states.
Up north, however, large-scale factories and other manufacturing operations were springing up in towns and cities on a growing basis, providing numerous job opportunities for many, including immigrants. The relatively sudden availability of new jobs was the biggest single pull factor in encouraging immigrants to settle in the North. The mid-nineteenth century was a period of rapid economic development in the Northern states, and to many escaping poverty in Europe, it seemed like the United States—at least in the North—was genuinely a land of opportunity that could provide immigrants with a chance to improve their lives.