Savings & Loans are quite often portfolio lenders, as are some banks.  Portfolio lenders generally promote their own portfolio loans, which are usually adfustable rate loans.  They will often pay more compensation to their  loan officers for originating a portfolio product than for originating a fixed rate loan.   You may also find that they are not as competitive as mortgage bankers and brokers in the fixed rate loan market.

However, it is often easier to qualify for a portfolio loan, so borrowers who may not qualify for a fixed rate loan may be able to obtain a loan from a portfolio lender.  A borrower may be able to qualify for a larger loan from a portfolio lender than he could obtain from a fixed rate lender.

Portfolio lenders also can serve as "niche" lenders because certain things are more important to them than meeting the more standardized underwriting guidelines of a mortgage banker.   An example would be a savings & loan which is more concerned with an individual's savings history than being able to fully document income, among others things.

If you apply for a loan with a portfolio lender and you are declined, you usually have to start the process over with a new company.

MORTGAGE BANKERS

If we are talking about the larger mortgage bankers, you can count on them having several strengths.   For the biggest ones, you will recognize the "brand name."

Usually, they are much better at promoting special first time buyer programs offered by states and local governments, that have lower interest rates and costs than the current market rate.  These programs are often available to buyers who have not owned a home in the last three years and fall within certain income guidelines. 

Mortgage bankers may have problems just because they are "too big" or they may operate like well oiled machines.

If you are buying a home and you need a VA or FHA loan and the development you are buying in has not yet been approved, they will be better at getting it approved than other lenders.

If your home loan is declined for some reason, many mortgage bankers allow their loan officers to broker the loan to another institution.   However, because your loan officer is so used to promoting the company's product, he may not be familiar with which institution may be the best one to submit your loan to.   Another reason is because wholesale lenders do not expect to get many loans from direct mortgage bankers, so they do not expend much marketing effort on them.

BANKS and SAVINGS & LOANS

Their major strength is that you will recognize their name.  In addition, they will usually be operating as a mortgage banker.  a portfolio lender, or both, and have the same weaknesses and strengths.