Of the three listed, I'd go for the Digitech. I have one, and while it's not the best out there, for the money, it's great. You can get a wide variety of tones out of it, and that blend knob works quite well for me when I want high distortion, but not just pure fuzz. I can blend in more clean and get the best of both worlds (at least as far as I'm concerned!).
As for the GK Diesel Dawg that was mentioned earlier, I also like that quite a bit as well. A word of warning on that one though, it is huge, and it has more gain than I'd ever know what to do with. I barely have it at 8:30 or so and it's still a little more distorted than I prefer when I'm really digging in. Its sounds are great and versatile, but that gain knob really piles it on fast. It sounds good with all the settings I've tried, i.e. I've heard basses that sound similar on many of the settings I've tried. IF you want the fuzz, or the heavy distortion, it's great. Takes a little more coaxing to get less out of it, and I'm still working on it a bit. It can be done though, so it's just up to you.
As for comparing both of them, they're both still on my pedal board. The Digitech seems to lose a little bass compared to the Diesel Dawg, but the Digitech doesn't have EQ knobs like the GK. Personal preference as always.
Unrelated, the Darkglass B3K and B7K sound amazing, certainly on my GAS list. No clue what your budget is, but they're way out of mine. Worth mentioning if you're wanting really high end stuff.
