I have 3 different brands of stick-blenders- a Hamilton Beach with plastic detachable wand, a Cuisinart with detachable metal wand, and a Kitchenaid with metal detachable metal wand.
My favorite of the 3 is my 2-speed Hamilton Beach with plastic detachable wand. It's the first stick-blender I ever bought for soaping and believe it or not, it is still going strong after 11 years of soaping with it (yes- the same one!). The higher speed option is the perfect speed for my soaping needs (not too fast and not too slow), and the bell housing and depth of where the blade sits in it are designed in such a way that it never needs to be 'burped' and never gives me get air bubbles in my batter.
The design of the bells on my other 2 stick-blenders are very different, as you will notice below (pictured L-R- Hamilton Beach, Cuisinart, Kitchenaid).
Notice how the bells on the 2 metal wands are quite concave/deeper compared to the plastic one, and how the blades sit higher in each.
Also notice how the 'air-slits' are designed in each. The plastic Hamilton Beach has an 'open-ended' design while the 2 metal ones do not (the slits are closed off). The 2 metal ones give me air bubbles and need to be burped, but the plastic one does not:
I only use my metal ones when I'm working with my glycerin liquid soap or HP where the batter can get pretty hot, but I use my plastic one for all of my CP batches where the temps can still get pretty hot, but just not as hot as the aforementioned soaps.