1) Why do you run blue cantrips?
They help to smooth out draws and give early plays to a color that doesn't really establish board presence till the mid-late game. Most U/x decks in my cube tend to care more about card quality than quantity and being able to dig to your high impact cards or making the most of each of your plays is essential to playing a game effectively if you're more of a control-y deck.
2) What makes you choose one over another?
Variety and power level of different effects. Brainstorm is the best if you have access to shuffle effects, but multiple Brainstorms can have diminishing returns in the same deck if you don't have enough. Getting Brainstorm locked into something like 3 lands absolutely sucks. In those cases, I would much rather have had a Ponder that could have shuffled me out of a bad position into something different. A lot of times you just want to dig deeper into your deck for something specific, so I like having cards that can scry away bad draws as well. I used to have multiple Brainstorms in earlier iterations of my cube but I've found that I just like having a variety of different plays available for my drafters. It's just more fun that way. I run the following cards:
3) Is the sorcery/instant separation vitally important and does it represent a noticeable change in power level?
Not really. I think Ponder is actually the most powerful of the cards in a vacuum due to the ability to re-shuffle, but Brainstorm becomes the highest quality of them when you can integrate another shuffle effect with it. Preordain is nice if you're just trying to dig deeper towards more impact plays and drawing you into a card that you've scryed towards is just really good for control decks. Opt is the least impressive, but it has fun interactions with stuff like
Stormwing Entity,
Young Pyromancer and
Monastery Mentor so it'll stay for a while. It really just depends on what the deck wants. A control deck is happy with most any cantrips they can get their hands on and all they really want is efficiency. Having one mana plays when you otherwise have only reactive spells ready to go is great for sculpting your hand and making the most of your future draws.
What mostly matters is the individual quality of the spells. I'd say for me it's something like Brainstorm = Ponder > Preodain >> Opt.