As for me, if both qplot and ggplot are available, the criterion depends on whether data is stored in data.frame or separate variables.
x<-1:10
y<-rnorm(10)
qplot(x,y, geom="line") # I will use this
ggplot(data.frame(x,y), aes(x,y)) + geom_line() # verbose
d <- data.frame(x, y)
qplot(x, y, data=d, geom="line")
ggplot(d, aes(x,y)) + geom_line() # I will use this
Of course, more complex plots require ggplot(), and I usually store data in data.frame, so in my experience, I rarely use qplot.
And it sounds good to always use ggplot(). While qplot saves typing, you lose a lot of functionalities.