The US Census Department has a great deal of information available electronically for the 2000 and 2010 censuses (and for the American Community Survey estimates which now serve in place of the "long form" census data) available on the American Factfinder site - and by the end of the summer much more should be up as the transition to a new version of the site is complete. Data for the 1990 Census is also available in electronic form, though generally only in raw tables. The Factfinder site's mission is only to crunch the 2000 and 2010 numbers. As you have found, a great many other sites out there offer this data in a variety of formats.
Data prior to 1990, however, is not available in electronic formats - or rather not in the easy-to-manipulate forms of the "born-digital" era. Tract or block level data is available for your city from 1950 on, in the print publications of the census which are available in pdf on the Census site or in a Federal Depository Library.
One thing to be aware of is that the nature of the data collected varies with each census, as do the boundaries of the enumeration districts/tracts. This makes it a labor-intensive project to compile the kind of comparative data you are after. For this reason I would suggest going to a collection that holds the print volumes so you can navigate through the tables more easily, to get an idea of what is available in them. It would be a good idea to call ahead to the library you intend to visit to make sure that they hold the relevant census volumes. A quick check of Worldcat indicates you may need to go to the State Library in Harrisburg rather than Scranton, though cataloging of such reports is inconsistent.
Not quite the answer you wanted, I am sure. Perhaps another librarian will know a better resource, and we'll both learn something.
ancestry.com
This is a paid resource, geared toward family history searches, but I have had some luck bending http://www.ancestry.com to other uses. They have searchable scans of early census and local data (no way to search by income, but you might be able to dig up what you need). They have a 14-day free trial.
Re: My own questions
I came across 2 tools that were helpful. It's important to keep in mind what another response said, that boundaries and other factors have changed from census to census. In my case, of the 3 counties I am researching, 2 did not exist until the mid-1800's.
'Historical Census' is the name for census 1790 - 1960.
The Historical Census Browser ( http://mapserver.lib.virginia.edu/ ) is pretty easy to use, but only goes to 1960.
The National Historic Geographic Information System ( http://data.nhgis.org ) does 1790 - 2000. It isn't very to use- I recommend reading the how-to before jumping in. With this, you request data and it compiles it in tables and emails it to you.