Decatur street is located near downtown on the other side of the CSX mainline. It was formerly one of the least desirable streets in town to live on. In the last 10 years that has changed dramatically. New townhouses have been built on formerly vacant lots and many old historic homes have been restored. It is now a rather trendy street to live on due to its historic character and convenience to downtown and the interstate.
Columbia St. is located in the North End of town. It is a one way street accessible only by going over the Frederick St. bridge, going up 3 blocks, turning around and then coming back towards the bridge. It is the kind of street you would never end up on unless you intended to go there. This isolation has lead to street to become ignored and it has fallen into decline. It was formerly a blue collar, largely Italian street, characterized by small brick rowhouses on narrow lots.
Now the street is mostly low-end HUD rentals and is one of the higher crime neighborhoods (by Cumberland standards.) I wouldn't say any place in Cumberland is "dangerous" or "unsafe" but living on Columbia St. would expose you to some of the worst things Cumberland has to offer. It is possible that Columbia St. could be gentrified like Decatur St., but for now I would definitely recommend Decatur if I had to choose between the two.
In answer to your second question. Go to
Cumberland Times-News - Homepage and check the classifieds for apartment listings.