So generally I think you're able to get more controlled delivery with a needle, while simpler without it. less tunable but less demanding of tuning. with a needle the taper varies the fuel air ratio relative to throttle position in the first parts of the throttle range. With a slide you're just enabling the same basic varying draw through the atomizer by affecting the venturi effect, essentially the velocity over the end of the atomizer affecting pressure to suck up gas. So notching it in the middle forces air flow over the atomizer and increases velocity at the tip drawing more gas up and behaving similarly to say, a thinner needle. it's different ways to hit the same goal, just the needle's a little more direct and rigidly controlled. so yea, needle is arguably more tunable, but also more sensitive to tuning, and honestly not a big diff either way.
The square bing is a unique one tho, I think they redesigned that to allow more flow with the same diameter, which coupled with the better vacuum and higher compression of the 44mm stroke D crank, and the better porting of the D cylinder, better ignition timing than the other versions and a bigger intake and exhaust all played into a more powerful setup. Very little of that power has to do with the lack of a needle, but it is the best square bing.
Swapping an SHA carb is usually pretty easy and advantageous just cuz the square bings are limited at 12mm and also have less available more expensive jets, however it runs into the frame on many models of sachs, sometimes you just can't use the choke or idle adjustment, sometimes you need to figure out spacers to get it to fit. With the round bing they don't really fit at all without a spacer to give more vertical clearance but at least on the flat intake ports of the C and D's it's pretty easy to fit a little riser in there. trickier w the angle intake on an A (and also unfortunately and foolishly angled on the Airsal kits...)
you can also just flip the D intakes forward, shave off the front corner fin, and slam on a 14 or 15 round bing. The intake's already about 14.5mm so works well w/ 14 or 15 carbs.
The 15 round bing is also my personal favorite of the available options here.