First is the cost. ADS comes in many flavors, bells, and whistles. The more you pay, the more bells and whistles come with the ADS. What you need to know is what you're trying to do and for how long.
For example, if you want comforts, you got the FortePlus. The vacuum system is great, the comfort is awesome, but there's a high cost in replacing consumables (comfort
strap rings, sleeves, and even cup sizes).
You got
SizeDoctor vacuum cup setup for its extender, but you also have to purchase specific cup sizes, consumable sealing ring, sleeves, and of course, upgrade the straps.
You got the
SiliStretcher2 and
StealthStretcher, which are pretty much the same as the previous two.
You also have assorted China/India branded knockoffs, very cheap, and tons of headaches. The so-called acupuncture/acupressure cups are notorious for keeping pressure going past -7inHg, and will slip/break off at 2kg.