Is the DT worth the extra $100? I wouldn't utilize the Drop tuning function all that much, mainly for "bass" tones, but its a pretty solid "drop tune" pedal for $100, if that makes sense. I'm slightly leaning on the side that I should get the DT, but $300 for a pedal is a little steep for me.

This is not considering buying one used.

Whammy: http://www.musiciansfriend.com/guitars/digitech-whammy-pedal-with-midi-control/150171000000000

Whammy DT: http://www.musiciansfriend.com/guitars/digitech-whammy-dt-drop-tune-guitar-effects-pedal

This post may contain my opinion and/or inaccurate information.

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It's all down to whether you think it's worth the money, if you think you'd get enough use out of it to warrant the price of the thing...

Just when I thought the Digitech Whammy couldn't take up anymore room on my pedalboard...

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Its a cool idea, be fun to experiment with. But if you don't think you'll use the drop tune part very much, why spend the extra money?
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This one says it's polyphonic. Maybe it sounds better? I'm pretty sure the regular whammy is just monophonic.
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Didn't think I'd use drop tuning much either, until I found how easy it was to shift from Eb to D std or E std while learning songs in diff tunings without retuning the gtr. Trust me, it's worth every penny. Plus it's true bypass so it's not a tone suck like the Whammy.
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From what I've heard, the DT is better than the IV in all applications on the pedal. If it is polyphonic and the IV is mono, that might be the explanation. The reviews I've read said that the DT was all around better and worth the money. I would grab it just because you never know when you might the drop tune function later. I mean, if you are buying a DigiTech Whammy, you're obviously gonna screw around with different sounds, so why not go all out?