The S30 is the better of the 2 cylinders as it is bigger, no other difference.
You can't carbonate lager in a pressure barrel though as a plastic pressure barrel is only suitable for low levels of carbonation like you would have in Ale.
For lager you either need to bottle or use Cornelius Kegs, which are made from Stainless Steel, as these can take much higher pressure which is required in order to carbonate lager to the desired amount.
Your valve could leak for 2 reasons.
1. You are trying to put too much gas in the Keg and the pressure release safety valve is letting excess CO2 escape.
2. The pressure release valve is not working properly and just letting all the gas out. I'm not an expert on them valves so I would either take it to the shop and ask them to test it or get one of these to replace it with.
http://www.the-home-brew-shop.co.uk/acatalog/Brass_barrel_valve_inlet_and_relief_valve.html