I have them both and the hardware sounds better.

Why?
I'll post an example of a kick but for now.

First impression they are very close but not until you play a double kick do you really notice the difference. The second kick is always different than the first. Meaning, if you have an aggressive attack set, the attack on the second is alway brighter with the software version. They are both excellent but once you have the hardware version, its hard to settle for the software. Hardware is warmer and more even at all settings. I think it also sounds a bit more open and better tails.

Something to be aware of, I have a TD 2 and am selling it because I need balanced I/O for the levels to match dead with my MixDream. The TD 4 is balanced so I am replacing it for that. I've heard they work pretty cool on vocals so who knows, two extra channels might be handy.

You can strap a jumper across the blue and ground of a TRS but its still not quite right. I wish SPL made the TD2 balanced, not sure why they do that, but, the TD 2 is quite a bit less so its a bargain. I'm guessing many would not even notice this but I am switching between channels with my MixDream and its annoying.

The Transient Designer is a must have.