If it's between firewire and usb1, then definitely opt for firewire.
If it's between firewire and usb2, then it varies from device to device. But quite often, despite the theoretical higher transfer rate, usb2 can be slower than firewire. Apparently it's because usb2 has higher overheads, whatever that means.
Be careful with what firewire cable you buy. Laptops usually has a 4 pin firewire port. Check. If your external hdd has a 6 pin firewire connection, then you should get a 4 pin to 6 pin cable to connect the two.
Also, a laptop firewire 4-pin port cannot supply power to the external device. So your external hdd will need independent power supply. Check the voltage requirements for the external hdd and whether it comes with a transformer. If it doesn't, you'll need to buy one which can supply the right voltage to the hdd when you connect it to the laptop using firewire.
Just checked the Maxtor site, if it's the Personal Storage DV5000 200GB model you're buying, it comes with an AC power adapter, and a 6 pin to 6 pin firewire cable. So you may need to buy a 4 to 6 pin firewire cable if your laptop has a 4 pin port.