I have both. The 5 speed backpack is very robust and although it says it holds 15 litres, it actually holds just under 19. Water will eventually interfere with the electronic controller though, I eventually had to bypass mine. A great price as well your getting. I paid almost £100 more from a different company.
The WCW backpack is also very robust but it will throw the water out at a high rate so you have to work fast. I fitted a wee tap on the outlet hose and this slows it down a fair bit which suits me.
No electronic circuits to worry about though, just tank, battery and pump. I prefer to use this backpack.