Criteria for silicone utensils
I submitted an edit to fix up your question because it sounded like a product recommendation.  Can you also provide more info, like what you would be using them for?

Here are a few characteristics I looked for in my recent purchase:


Thickness - thinner spatulas would be easier to slide under food
Stiffness in the handle - better for stirring soups, stews
Blade shape - for getting into the corners and curves of your jars, bowls, and pans


Of course, read up on reviews of the products you're researching.  These will attest to build quality, construction, ease of use for other customers.