7 Strength adds 1 DPS, regardless of your character's level. About 20 Agility (at level 60) boosts your melee crit chance by 1%, which amounts to boosting your average DPS (including Attack Power, both natural and from your weapon buff) by 1%. (Note that this means the value of agility relative to strength increases as your melee damage increases.)

If you have flurry (and it were working correctly for a full three hits), then a crit boosts your attack speed for the next three hits by 30% (assuming it's maxed, which if you have it, it almost certainly is). Fudging it, that's like getting 90% of an extra blow in over the next three hits, so your crits are really 190% extra damage, so 20 agility is then a 1.9% DPS boost. (Note that for sufficiently high crit percentages, your flurries will start proccing while you're still under the effect of other flurries, leading to diminishing returns.)

End result:

1 Str increases DPS by 0.143.

1 Agi increases DPS by 0.095% of your total DPS including attack power, if you have full flurry.

These two values become equal at 150 DPS. So, if you're breaking that with your attack power and weapon buff, agility is better. If not, strength.

However, this doesn't take into account the bonus that DPS added via strength already gets from your existing crit rate. To really figure things out:

Average DPS = (Base DPS + Strength/7) * (1 + 1.9*(Base Crit Chance + Agility/2000))

The end result is that you probably want both strength and agility, as their effects multiply each other. This is particularly true given the items available in-game, as it's easier to build up a reasonable level of both than a really high level of either.

I really like this analysis of Strength vs Agility for shamans, and if I every get to log into the boards with my main, with your permission I'll include it in the FAQ.

One thing to keep in mind is that using the windfury weapon buff affects this analysis. Windfury extra attacks count as attacks as far as flurry is concerned. So, if you get a crit, and then a windfury proc on your next hit, you will have used up all 3 of your flurry hits, and so really you only got that speed bonus for 1 hit instead of 3. Additionally, you can get a windfury proc on the second hit instead, which would lead to a speed bonus for 2 of the hits instead of 3.

This would change the DPS increase to 1.3% and 1.6% respectively, instead of 1.9%. The chance of windfury occuring on any single hit is 20%. Thus, the chance of the first situation occuring is 20%. The chance of the second situation occuring is 80%*20%=16% - 80% chance of the first situation NOT occuring, and then a 20% chance for a windfury proc. The chance of a full increase is thus 80% * 80%=64%.

The expected value of the DPS increase of a flurry proc when using the windfury weapon buff is thus 20%*1.3% + 16%*1.6% + 64%*1.9% = 1.732%, not a full 1.9%.

A similar analysis could be used to calculate the exact value when considering "lost" flurries that occur when a crit happens when a character is already flurried.

What about the defense agility adds? Since it's just "Strength vs Agility for shamans, shouldn't that be incorporated? Or am I mistaken as usual? :D

Yes, Agility, does add defense, but I was just looking at the effect of each on damage in this analysis.