The Denecke AD20 is a preamp and ADC with a minimum gain of 17dB, if memory serves. If you're running another outboard preamp, you're effectively running two pres and likely will need to attenuate the signal between the outboard pre and AD20. Pretty good sounding ADC in the AD20, though.
The Graham Patten ADC20 is just an ADC, no preamp.
Personally, I'd [1] skip the AD20 because of the double-preamp issue, [2] pass on the ADC20 - though it yields excellent detail, I find it painfully bright and harsh to my ears, and [3] go with the D100, a really nice ADC IMO.