It depends on what you are looking for as far as features. I tried them all before I made a decision. I originally bought the Denon s3000's and took them back for the CDX's and I've had them ever since. Here is my personal rankings:
1. Numark CDX - PRO's - 12" spinning vinyl platter, excellent scratch sound, exceptional response.
CONS- Slow loading time, pitch control isn't extremely accurate, slow mp3 reads.
2. Denon s5000/s3000 - PRO's - 7" spinning vinyl platter, good scratch sound, compact size, tight pitch control, . CON'S - on faster style scratches the digitized sound comes out, close button layout.
3. Pioneer CDJ1000/CDJ800 - PRO's - reliable (been around the longest), various scratch modes, tight pitch, tight response. CON's - non spinning platter.
4. Technics SLZ1200 - PRO's - Media card reader. CON's - horrible scratch sound, slower response, close button layout.
From my experience, for scratch heads, the CDX's can't be touched, they are the closest to turntables as you can get. I rock them with my Vestax TT's and it's only a slight pressure adjustment in feel. For mixing, I'd say the Denons or Pioneers, they have a tight pitch control and a lot of features and effects for dance. I'd say the best overall is the Denons because of the spinning platter and effects. I personally chose the Numark's cause I'm a scratch/performance Dj and they suited my needs better. The Technics I was not impressed with, and I lot of people I know took theirs back once they tried the Numarks.
As mentioned, Serato Scratch and Final Scratch are other options, I had Final Scratch 1.0, but a weak computer and it was cool, but it was still evolving and caused me a lot of headaches. From what I hear though, both are tight now. Like you said, go to Sam Ash and play around and see what suits you.
