The Pa3553 has an 1394a port, not b, i.e. 400Mbit/sec.
However this is considerably faster than USB2.0 which although advertises a similar speed, it is severely capped by overhead, which limits the transfer speed to only 25MB/s. On top of that seek speed of hard drives connected through USB is also severely affected.
Firewire on the other hand is much faster, all the advertised bandwidth is available (almost 50MB/s) and seek times are as fast as if connected using SATA/PATA.
So Firewire is always better than USB, if you can find a drive with a 1394 port, go for it instead of a USB2.0 drive.
However E-Sata is even better and the Pa-3553 has one of those too. Just make sure you buy a device that supports "power by E-Sata" which means it uses the E-Sata connection for data and the surrounding USB port for power. This can be tricky. (E-Sata doesn't provide power and an external power supply is required otherwise).