My confusion arose because both b and u flag seem to do the same task, except one outputs the compressed and the other uncompressed bam.
For example, what is the difference between other than the computing time?
samtools view -bS some.sam | samtools sort - some_sort samtools view -uS some.sam | samtools sort - some_sort
Most importantly, If u flag just for performance, why is it not default? what is the purpose having b flag?