That's right, this is the battle of the underwhelming 5-drop enchantments.
Pyroconvergence comes in at #69 among uncommons in Return to Ravnica, while Sphere of Safety rates as the 63rd uncommon in the set (out of 80). So, neither one is getting anybody hot and bothered. And really, that's appropriate. Neither of these cards is the kind of card that fits well into a drafted deck. I could see them helping in certain constructed decks, but that's not what we do. So which card is stronger in a draft scenario?
For me, the way to judge these cards is to look at what their effect would be at the point in which they'd be on the battlefield. Most likely, you wouldn't see either card before turn 4 even if they're in your opening hand. And since they're expensive cards, you'd probably be weighing them against other, strong cards that might be more effective. So, in general, I think you'd expect to play these cards on turn 5, with probably 3-4 cards in hand.
With Pyroconvergence, at that point you've got at most two multicolored spells in hand. If you had Rakdos Shred-Freak or Frostburn Weird, you've already played them. You're more likely to be sitting on Explosive Impact or Street Spasm. Pyroconvergence could potentially be useful if you're a strongly Izzet deck with lots of tricks and higher cost spells, but that kind of deck can get you in trouble.
Now let's look at Sphere of Safety. If you drop it in turn 5, you may have already had a chance to put down an enchantment or two. Security Blockade and Arrest are two particularly useful enchantments that help make Sphere of Safety much more useful, (in addition to themselves being vastly superior to either of these enchantments). Additionally, most draft decks you play against put their enchantment removal in their sideboard. So at least in most game 1s, once the Sphere is out there, it's out there.
Between Axebane Guardian and the various keyrunes, there's enough mana-fixing and mana-ramping that Sphere of Safety loses its mustard, particularly as the game progresses and mana becomes easier to find. But if you find yourself in a Selesnya deck without many ways to get to the late game, where you own, Sphere of Safety becomes a fairly useful card.
In the end, neither of these cards are cards you really want to play. They rely so heavily on the makeup of the rest of your deck that there are precious few circumstances when you'll want to employ them. But in a vacuum, assuming all things equal, give me the one that I can actually see working to my advantage.
Joe's Pick: Sphere of Safety