i have the greddy one.
have had no issues with it at all. its easy to use and works great.
has a great function that if you exceed a set boost pressure it knocks it back by a certain %. in my case if i go over 21psi it knocks it back to 17psi. had me stumped the first time as i had it setup at my first tune, and didnt realise it was set like that. no matter how much i turned it up it wouldnt go over 17psi, until i turned it back down under the threshold
also has a nice pretty screen , can have 3x setting, off, low and high.
has a great option that is a remote steering mount controller, think its like 150$ extra, but it straps to the steering wheel and has a button so you can switch between low and high on the fly. is wireless and a reciever plugs right into the back of the unit. great for drag racing as you can keep boost to say 7psi and only change up to 20psi only when the tyres grip. its kinda like scramble boost except you have control of when your boost level goes up.
no experience with the hks one sorry.