I am (was) using a Roland TD-6V. I don't think from what I have read that there is much of a difference between the TD-9 and the TD-6V other than appearance and maybe some of the sounds included or not included with the software.
The TD-6V has it's limitations, one being the limited number of sounds that can be assigned to each trigger. I think I only had 22 or 24 inputs available on the module, so I could only assign (without going in and changing MIDI note number on the module) 24 pads on the ZX to the module. The problem was with crossfading, each crossfade would take 4 trigger inputs on the module. When you start adding more crossfades, the amount of trigger inputs go down quickly. There is no brush sweep in the TD-6V, I don't think the TD-9 will have that capability either unless there is a sweep sampled, but then you run into the length of the sweep that is availalbe.
There are no faders on my TD-6V, I had to go into the individual drum with the cursors and change the volume for the individual drum. I used to split my stereo inputs, kick panned to the right channel and everything else to the left, that allowed me to adjust the kick seperate on the main board, but the rest of the kit was not individually adjustable by using a slider.
I think if you are looking to upgrade, I would go with the higher end TD-12 or TD-20. There are more trigger inputs (from what I have seen online and played with a guitar center), individual sliders for the main kit pieces, crash, kick, snare, toms, and ride.
I did have the ability to chain my kits together and can step through them using the cursor switch on the back of my ZX and I have added a Roland FC-300 MIDI foot controller to my aresenal allowing me to change kits without my hads leaving the Zen.
I have since upgraded to using computer based Superior Drummer 2.0 and there is night and day difference between the sounds and the control options and though I really like my Roland module, I really love SD2 and how it has brought my abilities and sounds to another level. I can't see myself going back to using my TD-6V for my Zen anymore.
I think I saw some electronic kits at Best Buy in their musical instrument section and they were using the TD-9. Guitar Center may have some as well. I would consider taking a trip down to one or both and take your Zen with you, plug it in and see what it sounds like. The guys at my local GC love it when I come down with my Zen and add new toys too it, even if it is just to hear what it sounds like.
Hope this helps,