I am far from an expert in the fundamentals of silver mining companies. I have been focusing on PAAS for two reasons. One is that Bill Fleckenstein is a director of the company. Bill is a hedge fund manager who does a lot of writing on the Internet. He says they have good management and I trust his judgment. And I know in the mining industry (as well as other industries) management is everything.
The other reason I own PAAS is that Bill Gates owns 10% of the shares. I think at the very least that's a plus.
SSRI may be just as good. I just don't know very much about them.