This post was requested by Kathryn in the comments of a previous post.
The two idols we're comparing are Idol of the Black Willow and Idol of Flaring Growth. Let's give them a quick once-over.
The Idol of Flaring Growth is an idol almost every Resto Druid owns by now, costing 25 Emblems of Triumph. It has a chance to grant a 234 Spellpower buff everytime Rejuvenation ticks. This proc rate is so high that often the buff will have a 100% up time with even one Rejuv ticking on a target. As soon as it procs, the buff is complete and immediate, and almost never drops off so long as Rejuv is rolling on a target. If Rejuv is rolling on two or more targets, you will never lose the buff.
The Idol of the Black Willow is an idol introduced in 3.3 and costs 30 Emblems of Frost. On every tick of Rejuvenation, the idol will grant the Druid casting it 32 Spellpower that stacks up to 8 times, granting a total of 256 Spellpower. It has no RNG; it will stack or refresh during every tick of Rejuvenation. It will have a 100% uptime so long as one Rejuv continues to tick.
It seems pretty obvious which Idol is better, as it should be. The difference is, however, only 22 Spellpower. While some argue that the ramp up effect makes the Idol of the Black Willow less appropriate for PvP, it is more reliable and less reliant on RNG (if we're talking about a single Rejuv). It is true that, once it procs, Flaring Growth has no build-up phase, but Black Willow should be easy to stack with the continuous Rejuv that should almost always be on a Druid or their partner(s).
However, as was stated earlier, the difference is only of 22 Spellpower. The Idol of the Black Willow is not a make-or-break piece as Flaring Growth was in the time of ToC. It is not a big deal to wait to pick this Idol up later, after you have completed more expensive purchases. Especially now that Emblems of Triumph are everywhere, it is more prudent to buy Flaring Growth and wait until you have no other uses for your Frost Emblems to invest in Black Willow.
So, in summary, Black Willow is obviously better, but not so much that you can't wait on it while purchasing other items with your Emblems of Frost.