Verified answer

So i compared both phones with the help of GSMARENA. Most of the features are identical, only a few differ

1. Desire C as a bigger Internal Memory (4 GB) compared to Ace Duos (3 GB) But that can be solved by use of an external Memory Card

2. Desire C comes preloaded with Android 4.0 (Ice Cream Sandwich) It's not the latest version but it is way cooler than Ace Duos Android 2.3 Gingerbread

3. Ace Duos has a faster processor (832 MHz) when compared to Desire C's 600 MHz. This is not a very big difference though

4. Ace Duos can take 2 Sim Cards, Desire C Can't

All the other features are the same. Here I think it is gonna depend on the Manufacturer you like. Is it the HTC or Samsung?

But if it were me, I would choose the Samsung Ace Duos s6802. Why? Even though it has a lesser advanced OS than the HTC Desire C, that can be taken care of by the various Custom ROMs out there. It has a faster processor than Desire C's, that can't be taken care off, plus if the processor is at 832 MHz without being clocked that means you can overclock it to 1GHz or even more.

Another big thing to consider the Duos is that it can take 2 Sim Cards unlike the Desire C which can only take 1 Sim Card

:D