My house rule is that Sure (and Careful) Strike do [W]+Str damage (+Dex instead for Careful used at range).
I am comparing it to Valiant Strike (Paladin at-will; Str +1 per adjacent opponent vs AC; [W]+Str damage) and Piercing Strike (Rogue at-will; Dex vs Ref; [W]+Dex damage). Valiant has an attack bonus that's at least +1 (unless you're using a reach weapon, but then you're doing it wrong) and quite often will be even higher, and still gets to add stat bonus to damage. Piercing Strike is one of the few powers that get the Weapon keyword and target a non-AC defense, which are supposed to be on average 2 points lower than AC according to the monster design guidelines, and also gets to add stat bonus to damage.
So I think Str+2 to hit, [W]+Str damage is a pretty balanced deal for Sure and Careful Strike (substituting Dex for ranged Careful).
I have seen some people argue that getting attack bonuses in 4e is really powerful, and is worth giving up the extra damage for. I disagree, at least in this case. Attack bonuses are great when you can use them to set up a special attack and make certain that it hits. If I could get a +2 to hit with my Thicket of Blades power (Close Burst 1, Str vs AC, 3W+Str+Slowed (save ends)) at the cost of losing Str bonus to damage, that would be worthwhile, because the Str damage is pretty small in comparison to the 3W, not to mention the Slowed rider effect. But it's not worth giving up your Str bonus to damage when you're only doing W to begin with and there's nothing accompanying the attack.