All other things being equal a 7200 RMP drive will be faster then a 5400 RPM. There are two things affected by the RPM of a drive.
Seek time, on avarage a 7200 rpm drive will spend less time waiting for the head to reach the sector on the disk it needs to read.
Transfer rate, the speed data passes the read head. There are two factors that effect this, the RPM of the drive and the density of the data on the disk.
If you have two 20 Gb/platter drives a 7200 will beat a 5400. If you have a new 20 Gb/platter 5400 RPM drive and an old 1Gb/platter 7200 RPM drive the 5400 RPM drive will win by a large margen, transfer rate could be 3 or 4 times that of old drive.