You can use rekordbox 5 or 6, there's a license for 5 in the box, and 6 unlocks when you connect the DDJ. Nobody's being forced to update, it's entirely user's choice.
The main difference in the FX are these:
The only option is subscription for those, but you also gain access to the additional Creative features.
Are you better off with Traktor? Not with the DDJ-800, the software won't have full use of the hardware.