stock NT carb = 14mm, no spring loaded float valve (which can cause gas overflow)
Speed carb = 13mm, spring loaded float valve
The other main difference between the two carbs are their needles:
The following graphs are from my jetting calculator and they show how the jetting is from 1/8 slide open to wide open throttle. Ideal is the red graph staying at 1.0 all the way (like a horizontal line). At low throttle openings a 10% variation is OK. The jetting calculator is just set up for engines with reed valves although its graphs may have some relevance with piston port intake engines also although the needed jet sizes are different.
The stock NT carb is too rich at 3/8 slide opening for the 66cc engine when the needle clip is at the top position. The graph is even worse at any of the lower clip positions. (Read the notes on each graph.)
Here it is corrected a bit by increasing the slide cutaway to 1.5mm (by grinding it down .5mm) but still it is too rich around 3/8 slide open. That can be corrected more by increasing the cutaway more but then it would be extremely lean at 1/8 slide. So this tells me the NT is somewhat hopeless.
The Speed carb on the 66cc engine is too rich up to 3/8 slide open:
Here it is corrected to perfection by increasing the slide cutaway to 2.25mm and setting the clip to the 3rd (middle) position. Main jet size needed to be reduced a tad also. (see notes on graphs)
The NT carb for the 48cc is just a tad lean at 1/8 slide with the needle clip at the top position. You would definitely have to use the choke to start it but then it should be usable.
At lower needle clip positions it becomes too rich at 3/8 slide.
The Speed carb on the 48cc is a tad too rich at 1/8 slide opening. (middle needle clip):
It is corrected by increasing the cutaway to 1.5mm:
If you make these changes then although you can't adjust the idle mixture you can get by with these carbs if the weather isn't too extreme. Which carb is better? I'd definitely go with the Speed carb because it can be corrected to perfection and because it has a spring loaded float valve to prevent overflow on bumpy roads.
To find the right Dellorto main jet to match the jet diameters specified here just multiply the jet hole diameter by 100. Example: .91mm hole diameter x 100 = #91 Dellorto main jet