Other than competition models, there are basically two styles: The HKS and Safariland. The HKS has a knob that is turned to release the cartridges, and the Safariland releases by pushing the entire body of the loader forward towards the cylinder.
I use both styles, but tend to prefer the Safariland Comp-1, which is really compact compared to the HKS. The Comp-2 is about the same size as the HKS, and has more gripping surface.
Speedloaders are cheap enough that you can buy one of each type and see which one you prefer.
For the N-frame Smiths you are pretty much limited to the HKS. But in anything .38 or .357 you should be able to get the Safariland speedloaders.