Deidara's art is unique. Sasori's art is just another puppet jutsu.
I prefer Deidara more. His drive and persistence is very interesting. I am actually very intrigued as to how he grew up. It seems like he always wanted to be noticed or recognized based on his skills and if he didn't get that recognition than he would prove it through necessary means. Deidara's style of attack is bizarre but at the same time cool. He took out the Kazekage on his own turf in one fail swoop and he didn't even break a sweat. It's not only his strength, but also his personality, especially around Tobi. Knowing a villain or bad guy, it's always good to see the Akatsuki play around without being serious all the time.
When it comes to Sasori, I was never fond with the outcome of his death caused by two insignificant kunoichi. It just made his overall diploma seem tiny and he could have killed them at many times if he wanted to, but of course a certain someone had to be saved by plot. When it comes to Sasori's backstory, it's pretty good. The best thing about Sasori was that he was able to manifest himself into an actual puppet.
I've always preferred Deidara out of the two. His speech. His art style. His personality. His goal. His fighting style. Everyone of these attributes has been a step ahead of Sasori.