If so, a bluetooth dongle can be much more convenient as you can use it for a lot of various uses (mice, phones, headsets, etc.) however data-heavy Nokia PC Suite functions like back-up will be magnitudes slower than if you used a USB-cable. Synchronise can take a while, too. So if you can only afford the luxury of the cable or dongle and plan on backing up the phone, contacts, content, etc. I would recommend the USB Cable.
As I'm lucky enough to have a Nokia that has a mini-USB connector and bluetooth built-in, I tend to use both. Backups and large transfers of data over USB Cable and for quick things such as 3G data I use bluetooth.
If the dongle is something else completely, my apologies :P