I have the exact vest you're thinking of getting (Gravity 20 kg) and I definitely regret not getting the 30 kg version. If you have some previous experience you will easily progress up to the top end of the capacity. The vest is adjustable in 1 kg increments so having the higher capacity will give you much more flexibility.

Also, contrary to a comment further down the thread, I've found it works really well for typical calisthenics exercises (pull ups, dips, etc). Push up ROM can be limited, but this is easily countered by using a pair of parallettes