Rogue Gear spreadsheet (see
Rogue Gear Spreadsheet) has a reasonable model of Thundering Skyfire assuming 1 PPM, and finds it to be *way* better. Brief summary of why:
Assume, for the moment, that it only actually procs once per minute (note that if it's actually 1 PPM, it'll proc about 3 times a minute on average - but lets just compute this based on 1 actual proc per minute). That means you gain ~25% haste for 6 seconds - roughly a 2.5% increase of white damage over not having it. On a sustained dps fight, white damage typically encompasses north of 60% of total damage done, so TSD increases your total damage by roughly 1.5% based on damage alone.
In reality, it's going to be somewhat more than that; more haste effects means more weapon attacks, which means more procs - and most rogues these days have several powerful procs (Poisons, Combat Potency, Double Mongoose, and TSD itself, if nothing else. And some will also have Dragonspine Trophy, Hourglass of the Unraveler/Tsunami Talisman/Romulo's Poison Vial, and so on). So it's going to give some amount of extra proc damage as well; hence, actual damage granted could easily be has high as 2%.
Meanwhile, a 3% boost to crit damage is going to increase the damage of ~30% of your white and yellow attacks by 3% - less than a 1% increase, affecting about 90% of your attacks (leaving out damage from rupture, poisons, and other procs) - so maybe a .8% damage increase overall. So in order for Relentless Earthstorm Diamond to be better, 12 agi has to give roughly a 1% damage increase. Grinding some numbers in your spreadsheet of choice will show that this is not the case, by a longshot. As such, TSD winds up being more damage.
Long story short? Haste is really powerful.