Having used both solutions, the Renew is by FAR better in every respect with exception to complexity of the install. The Stenner is a simple install however you won't have the same flow out of both heads as there is always a slight variance. The end result is that you will have a bit of salinity drift unless you can use a valve inline and get the flow dialed in between both heads identically. The flow rates will vary for a number of reasons including differences in the wear rates of the tubing between the two heads as well as head and draw pressure differences between the NSW side and the waste water side. The Renew is always precise as long as the metering bins are mounted level.
The Stenner is compartively slow where as the renew can change a gallon every 2-3 minutes.
The Stenner can only operate via a timed rate where you have to calculate how long it takes to change a gallon where as the Renew is based on gallons. The Renew can change out 1-99 gallons in one sequential run or daily or weekly. All of which is user selectable. If you have an emergency and need to change out a large volume of water or simply want to do a large water change, it's as simple as determining how much you want to change and pushing the buttons.
The Stenner pumps are very noisy. Like a coffee grinder. The Renew is virtually silent.
Hope that helps.