Twin strike is fine as-is. Careful Attack should definitely not do more damage - that entirely defeats the purpose of the power (a ranger already has a bunch of high-damage abilities). The attack bonus should simply be increased, say to +4, or, if you're very conservative, to +3. A human melee ranger is liable to take it anyhow, so there is an argument to be made that you don't want it to be too powerful. Still, I doubt +4 is particularly overpowering, so that's my vote.
Sure strike, the fighter equivalent, is weak as is, but playable, because of class differences - it doesn't have a twin strike competitor for minion slaying goodness, and a fighter is less single-foe centric role-wise, and it meshes well with heavy blade opportunity and heavy blade opportunity (which is a common build). I'd be happy with a change to +3, but +4 would be to high IMHO.
My preferred errata: Sure strike raised to +3, and careful attack to +4. If you're looking for a different style improvement, have careful attack instead grant a +1 power bonus to AC/reflex. Neither power should deal more damage than now; as is, the damage penalty isn't particularly high anyhow, and by improving the damage, it simply becomes a better basic attack which is boring.