What criteria would you use to judge whether one is better than the other? Price? Trust?

Also, what kind of SSL Cert are you looking for? There's a big difference between EV certs, and say a single domain cert, wildcard, multiple-domains, etc.

For single domains, I've had pretty good luck with Comodo through Namecheap. It generally costs 9.99/yr for Single domain, and I've even used their multiple-domain certs.

but yeah, more info would definitely help give you a better recommendation.