All things are not created equal, and these adapters are no different.  Basically, one requires more fabrication than the other, however neither are true "bolt-ons" when used with pre '66 Ford 144 - 200 sixes.  Linkage fabrication is required to make either of them work.  The simplest route is to convert to a cable actuated throttle and eliminate mechancial linkage issues altogether.

Side by side comparison of the two adapters:

In the photo above, you can see the Stovebolt adapter has a larger manifold opening than the Clifford adapter.  You can also see my rough sketch on the Clifford adapter showing how much would need to be remove inorder to "port match" 1.5" manifold opening.  In my opinion, this is necessary to realize the full potential of the adapter, however, Clifford Performance does not share my opinion.

***CAVEAT: After I received my 08-1650S adapter, I contacted Clifford Performance and spoke to a tech rep who told me, "Hog it out if you want, it won't make any difference." Anyone modifying these small sixes knows that increasing the manifold opening leads to significant flow improvement. Hence, my disagreement with their position. I visited Clifford's website 12/03, and it appears as though the small port adapter has been redesigned and may now have the correct manifold opening. I, however, am not willing to spend another $68 plus S&H to find out!***

In this photo, you can see another disparity between the two adapters. The Clifford adapter has a generous carburetor opening, while the Stovebolt adapter is much smaller. My Stovebolt adapter is 35.5mm wide at one end and 36mm at the other with an overall length 76.5mm. With a thin carb gasket, the throttle plates of my HW 5200 scrape the side of the adapter. A 1/4" thick carb gasket alleviates the interference. The Clifford adapter measures a consistent and generous 45mm x 92.5mm, and there is no throttle plate interference with a thin carb gasket.  

Here you can see the difference in overall height between the two adapters.  One advantage to the Stovebolt adapter is that it properly orients the carburetor (when used in conjunction with the original Ford adapter), with the float bowl facing to the front of the car.  This elimates fuel starvation/flooding during turns.  The down side is the increased height.

The ideal adapter would allow the removal of the original Ford adapter like the Clifford unit above, but have a rotated top allowing the carburetor to be positioned like the Stovebolt adapter below: