I use 3 sites – Ancestry, ScotlandsPeople and FamilySearch
Ancestry: I use Ancestry for the censuses, because all the people in the household are listed. Although the images are on ScotlandsPeople, the index on ScotlandsPeople only shows the person you looked for, and not the other members in the household, and it can get quite expensive if you have to view every image to make sure which one is for your ancestor.
FamilySearch: I use FamilySearch when I want to find the children of a couple. Although you can do the search on ScotlandsPeople, the index on ScotlandsPeople only provides the baptism date, while the collections on FamilySearch (and Ancestry) provide the birth date as well. I prefer to use FamilySearch for this, as there are less irrelevant results than when I try to use Ancestry.
ScotlandsPeople: I use ScotlandsPeople to search for deaths and for the other records that aren’t available on Ancestry or FamilySearch. I also use ScotlandsPeople when I want to focus my research. ScotlandsPeople has a better filtering system, so that you can limit your search to a parish or county, and you can pick a date range. It also has a range of options to search for variants – wild cards allowed, names beginning with, phonetic search, name variants. And of course, I’ll use ScotlandsPeople to get the original images. I get a few at a time, when I can afford it.