So there seem to be a number of these glass "manual" vapes, the Elev8r, the Vaphit, the Terpsicle, the Quartz Cap, the 18mm (and apparently 24mm) Killer (I'm sure I've missed some) - what distinguishes them? All I have of these is the ur ball vape, the DC Elev8r. Which of these are good for small bowls?
There is basically two designs in the vapes you cite here : the double-flow design originated by the Vaponic and the glass ball-vape design which, in his manual iteration, comes from the Sand Castle. (Which is itself a hybrid of a glass symphony and a banger.)
You can add a third category represented by the biovapo, roasty, nameless conduction, etc... and a fourth category that are glass bricks and vaporgenies.
The double flow design gives a hybrid heating profile between convection, conduction and radiation. Works well with flower and hash, pretty efficient. Size, thickness of the glass, air intake design, change the balance between conduction and convection, the heat retention. At 40€ I strongly suggest you to get a Vaponic, it's a very balanced design.
Ball-vapes and bricks / VG are basically pure convection and biovapo / roasty pure conduction.