Your maths is a bit wonky here.I think there are some assumptions on your part that aren't really clear to me for example saying the to hit chance is equal seems odd to me when one power requires 3 to hit rolls to work to its full effect and the other only one.

It's true that one needs 3 hits to work at full effect. But equally it has 3 chances to work at least at partial effect.

Let's call the hit chance H/20.

Then the chance of Tide of Iron hitting is H/20. So the expected damage from Tide of Iron is H/20 * (W + STR + fixed bonuses from feats, items etc).

The expected damage from RoB (2 attacks) is equal to the sum of the expected damage from each attack:

(H/20 * (W + fixed bonuses) ) + (H/20 * (W + fixed bonuses))

Collecting the common factor, this equals:

H/20 * (2W + 2*fixed bonuses)

Notice that both expected damage expressions have a common factor, namely, H/20.

If we want to compare the ratio of the two powers' ouput we can cancel out this common factor, and get a ratio of:

Tide of Iron : Rain of Blows = (W + STR + fixed bonuses) : (2W + 2*fixed bonsues)

Rain of Blows will do more damaeg than Tide of Iron any time that (W + fixed bonuses) is greater than STR. Which, as [MENTION=87792]Neonchameleon[/MENTION] points out, is likely to happen as soon as any item bonus is factored in.

And once you get three attacks, the "2" in the statement of the ration becomes "3" - which means that Rain of Blows will do more damaeg than Tide of Iron any time that (2W + 2*fixed bonuses) is greater than STR. Which will be true at all levels.