It seems that MBAM and Super Antispyware always come up as recommendations.
I only want to add one to my computer.
Which would be best--MBAM or SAS?
Best is very subjective and both programs are actively being developed, both have slightly different features also. So as has been said why choose one, when using the free version they are on-demand and use limited resources, primarily the HDD space used to install and that is negligible.
I use SAS Pro, primarily because I used that for some time before I came across MBAM and you should only have one resident anti-spyware, in the same way you should only have one resident anti-virus.