Don't rely on blocking too much in pvp, a lot of people use weapons that inflict bleeding, or weapons that tear through stability.
I'd recommend the heater shield for pvp, it's the lightest shield with 100% block (I think it's like 2.1 lbs, it's been months since I've played this game, but I know it's light as hell...) and it has usable stability for pvp if you use it sparingly. (Also, it can obviously parry as well).
If you have enough wiggle room to use the silver/block knight shield, go with those, but if you need to better meet a weight requirement, go heater.