CAPCOM is doing a great job in hyping SSFIV and we already know it is packed with nice features. In the other hand, Tecmo is not giving us enough info regarding DOAD's features. We know it will sport a crossover with Metroid but not exactly how and it will use the 3DS Pass option, although, again, no details are given.
I am pretty sure SSFIV will be more popular but I will give you two reasons why you should consider DOAD:
1) It runs at 60 fps if you turn off the 3D, whereas SSFIV runs at 30 fps in any mode. Even running in 3D, DOAD still calculates the motion in 60 fps (although it only shows 30 fps). As such, animations and gameplay can potentially be better.
2) DOAD uses four buttons to control the attack, whereas SSFIV uses six. I believe that may make DOAD much more comfortable to play than SSFIV, but that's up to each player individually.
In the end, I believe, you cannot be wrong with any of these games. However, I would wait till release and try out them in a store before choosing your poison.
