I've been using
a Line 6 M9, and been really happy with it - at least for chorus delay and reverb (although I mostly use the reverb in my Renegade). I've been wanting to try out a Nova System to compare, but haven't yet. Conventional Wisdom would say the Nova System should give more "studio-quality" delays, choruses and reverbs, being that it is from TC. OTOH, the quality of the L6 verbs, chorus, delay is really good - both the M9 and the M13 have the same effect algorithms that are in
the Line 6 MM4 and DL4. I like the "stompbox simplicity" of the M9 - it works exactly like a stomp box - call up the model and tweak the knobs. You can set it so that it automatically saves any knob tweaks (just like how it would be with a real pedal), or so that you have to manually save. The Nova System looks pretty straightforward too, but seems a bit more deeper and menu-driven. I do want to give it a try though - the Pete Thorn video of the Nova System on the TC site is a pretty good selling point.
The other nice thing about the M9 is the size. The M13 and the Nova System are not as compact.
I think the weakest link on the M9 are the drive/distortion models, whereas the drive circuit on the Nova sounds pretty good, at least in the demos. I think the M9 drive models are ok if you want a Fuzz sound, or something lo-fi and "effect-like", but if you want a nice full drive/boost/distortion, I'd stick with individual pedals. I don't really use the drive models on the M9 - only the compressor sometimes and the EQ. On all of these points (EQ, compression, drive), I think the Nova definitely has the edge, at least from the demos of the Nova I've heard compared to what I hear out of my M9.
It would also be nice if the M9 could be powered from the Voodoo Pedal Power, but the Line 6 jacks on it say 250mA max, and are designed for the MM4, DL4, DM4. The M9 is rated 2A, so no go.