The Fadal machines all take a cartridge type spindle, they can have either grease pack bearings or air/oil type. The diffrence is with air/oil, you can run continuous 10k rpm or higher. Either spindle can also have ceramic bearings installed. If ceramic bearings are installed in a grease pack spindle, you can also run continous 10k rpm. Only down fall is if the spindle takes any type of hit, the bearings will suffer. (they also tend to be more expensive when installed).
If the machine does not have an air/oil spindle system installed, and you want to "upgrade" to an air/oil system, you can add any where from $4k to $6k onto the replacement.
As far as which one is better, it really depends on what you are using the machine for and your max spindle speed.
If you cut steel or other hard metals, I would advise against any spindle with ceramic bearings. On the other hand, if you do mostly finish cuts, or carbon/electrodes, then high speed may be for you.
Hope this helps.