I would not pick the Radium (unless it supports sysex) but yeah, a control surface is nice. I'm mainly a rackmount synth guy so for me it is almost required since none of them have anything close to usefull interfaces and software controllers do not cut it for me.
I would go for a rack module and a controller over the XP keyboard since you can use your controller for everything, while the XP keyboard only has 4 assignable faders which is not that useful for programing (should be sufficiant for live filter sweeps etc). A good controller will allow you to manipulte most if not all of the synths parameters when working on patches (or live) in a tangible manner vs. digging through endless menus and setting prameters manually.
If that is what your interested in, I would study up a little on midi since programing controllers can be tedious if you really have to dive into the sysex aspect of it. But then again many keyboards make it easy by just assigning CC values (Midi controller msgs) to everything usefull so break out the manual and check what you will dealing with.
I am a fan of novation midi controllers since they really do offer a ton of control options AND the programmer is the easiest I have seen. The M-Audio/evolution ones are pretty basic and are limited to assigning CC channels to controls only (AFAIK, I didnt check)