What I thought Onyxnox was talking about was cheaper clear stamps. If they are old or cheap, they rip more easily. To clarify my vocabulary in case your's is different: there's unmounted rubber that you mount on your own cling foam. There's also rubber already mounted on cling foam, and there's clear stamps--best quality are made from polymer; worst quality are silicone or plastic.
Here's a link to a short article talking about clear stamps: http://www.stampavie.com/about-clear-stamps.php
. And another: http://www.blockheadstamps.com/TechniquesPages/clear-stamps.html
Again, I prefer rubber, and at this point buy bare rubber almost exclusively and mount it myself on cling foam.
But, again, it depends on how you plan to use the stamps, how long you plan to keep them, how much you want to spend, and how much space you have for storage. There is no one best stamp, there is only best for you.
It's very wonderful to share what I've learned. Thank you for the opportunity.