630 has Bluetooth, but misses FM transmitter. Not something you would use every day.
In my opinion, the x20 and x30 are the best if you do not care for Live Services/HD traffic (paid subscription) since it is easy to connect a TMC transmitter to them (USB or jack, but then part number 0013 only).
Differences between x20 and x30 are cosmetic only (although on the 730 the 2GB flash memory seems to be improved on). All have an SDHC card slot and will accept cards of at least 16 GB. Current navcore is 8.351 which does support ALG, IQroutes and all.
They are also the device of choice for running truck navcore 8.395 since the Go 7000 truck is essentially a 720 with GPRS modem. Having said that, the Go 9000 is a 940, so x40 devices will run truck navcore too.
Differences between 5xx, 7xx and 9xx:
520/530 have 1 GB internal, FM, Bluetooth and come with regional maps from the region they were bought in.
720/730 have 2 GB internal, FM, Bluetooth and come with continental maps of the continent they were bought in.
920/930 have 4 GB internal, FM, Bluetooth, remote control and EPT and come with maps of Europe and US/Canada resp AUS and US/Canada.
EPT: Enhanced positionin technology. It uses an accelerometer and gyroscope to measure speed and direction continuously in order to keep navigating in tunnels, between highrise buildings when you lose track of GPS satellites.
After having collected all the above info over the last months, I have searched for and found my new Tomtom, a Go 920T with TMC receiver. This weekend I will start playing with a class 10 SDHC card on which I will try navcore 9.051 and 9.101.