Um, same thing. If they can't use Rage Strike until 5th Level, they would be minus 1 at will power until 5th Level. So technically, they'd have only 1.
Rage strike is a class feature, NOT an at will attack. You get your two at wills AND you get rage strike, just the same as how a wizard gets two at wills AND 4 cantrips. Rage strike is a barbarian cantrip (.....well not really, but its the same idea)
consdier the suggested build.
Suggested Feat: Weapon Focus
Suggested Skills: Athletics, Endurance, Perception
Suggested At-Will Powers: howling strike,
recuperating strike
Suggested Encounter Power: avalanche strike
Suggested Daily Power: bloodhunt rage
NOTE: he has two at wills, neither of which is rage strike. (compare to warlock build advice, its "chosen for you" at wills are listed)
Rage strike isn't nearly as klunky or blatantly unfair as people seem to be saying, IMO. Its a simple idea. You are in rage. Burn a different rage to do this attack. This is so you can do big attacks, while keeping the rage you have. If you don't like it, don't use it, and you will never suffer, but if it wasn't there, it would be annoying that you couldn't use another daily level damage without leaving the rage you want to stay in.
As for not using it until level 5: THIS IS NOT A PROBLEM. Seriously. You can't use it before level 5, BUT you don't need it before level 5, you won't even want to use it before level 5, and its not such a big deal that you will in ANY CONCEIVABLE WAY be inconvenienced not being able to use it before level 5. However, what if, say, half-orcs can rage as a racial ability. Well then orcs can use it at level 1. What if there are magical items that let you rage, well then you can use this ability before level 5. Anything is possible, so the fact you get Rage strike from level 1 means that if you somehow enter a rage via non-class methods, you can still use it. I seriously can not see anyway in which this is, or could be, a problem.