Personally, I'd recommend Dual Strike first. Though the second one has a bit more polish and an impressive presentation, the story and dialogue are...well, they're pretty bad. The first game was funny because of just how over the top the characters and situations were, but the second one just didn't work. Gameplay-wise, it's still great, and again, it looks great. I just wish they either had better writers or stuck with the more campy nature of the first.
I enjoyed days of ruin but IMO it lacked the "fun-factor" which all the other games in the AW series provided. Whilst DOR does have online play, the way it's been set up doesn't bring much excitement and it severely lacks in modes outside of the campaign (and even that lacks a hard mode). Having said that, to an extent it is harder as the co powers have been removed so if you want more realism/challenge then go for that.
If I had to pick, I'd go with dual strike to begin with. However that one is a sequal to the 2 gba games (although not knowing what happened back then doesn't mean much - just that you won't be familiar with some of the returning Co's).
ah okay. the only reason i considered starting days of ruin first was for the online factor. didn't want to miss the train completely. but if the online mode is not that fun anyway i guess i'll just start with dual strike.