Hands down the original. Syndicate has always been about the immersion for me and SW had nothing close to the same feel. Personally I have always felt the SW concentrated on graphics over gameplay.
Immersion was probably the biggest problem I had with SW initially: it felt like an action spin-off, environments were simpler, more sci-fi elements were introduced...but looking past that, I'd say the gameplay was better in almost every respect.
I don't think SW was more focused on graphics (it looked markedly worse than the original). Playing it now I feel they did everything they could to further the gameplay and to get the most out of the technology. It was also created by a new team with a lot to live up to. Sure, mistakes were made, and it probably didn't live up to the original in terms of how revolutionary and immersive that game was for it's time, but it's still a great game and a good sequel.