The effective bandwidth of the USB interface proves to be a bottleneck for 2.5” hard disk drives with a spindle rotation speed of 5400rpm, but it is not for 4200rpm drives. Our tests of the Fujitsu Handy Drive show that performance of 4200rpm drives is largely determined by their own capabilities rather than by the interface. The 1200rpm difference in the rotation speed is easy to see and the slower HDDs deliver lower performance indeed.
So, if you need the highest possible sequential read/write speed, you should find an external enclosure with a FireWire or eSATA interface and install a 7200rpm HDD into it (7200rpm drives of 2.5” form-factor are already available in shops). Keep it in mind that this interface has minor problems with small files and multiple threads, yet these problems are not as big as to make you worry about them.
But if you are looking for a large-capacity external HDD with low power consumption and low price from a renowned brand, and its speed characteristics are not very important, you may want to consider the Fujitsu Handy Drive as a possible buy.