Well, when considering buying mathematical multiplication from one of the many mathematical extortion services, since you can pretty well trust that any of these guys actually have computers capable of multiplying numbers, so you have to go on the more intangible services they are offering.
If you look at comodo.com, you can see they can't really tell what they want to offer you. If you stay for a while, they will show you pictures of a father that is dissapointed in his son, racks of servers that seem to be turned off, their, CEO that is trying to hide the fact the he is psyched about how much money he is extorting you for, etc..
If you go to the verisign website, you will know that they mean serious business about multiplying numbers together for you. You might see them advertising with the ubiquitous curly brown haired woman, but she won't be laughing at salad.
You might choose godaddy if you feel like these other number mutiplication providers aren't providing you with enough advertising during the checkout process, of if you think that seeing semi undressed photos of Danica Patrick is important to your number multiplying service provider experience.
With an https website, there might be other factors like "do I want to turn their url bars green and shit" which will cost you, of course. because it like turns green and shit. Or "do I want all of *.example.com" to work. Its not like the multiplying of numbers costs them more to do because there was a *. But shit, *s are awesome, you should be willing to pay as much as 10 times a much for one of those certificates. Multiplying with numbers when * is involved is just awesome.
If you are needing to support clients which are using like really old software from before they supported chained certificates, then you might need to buy a certificate from one of the big boys like verisign. But we'd be talking OLD, like "Michael Jackson before plastic surgery" kind of old clients.
Otherwise, you just need some numbers multiplied together, just trust that pretty much anyone that has wedged themselves into the extortion racket is able to actually multiply numbers. Perhaps to stop this racket, contribute to projects like monkeysphere, which try to put this simple mutiplication back into our own hands.