Only in certain circumstances is precision better than advantage, and, honestly, you're unlikely to encounter those circumstances in normal play.
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However, the utility of being able to use the +d8 after seeing the d20 roll significantly increases it's use efficiency, but that also increases observer bias in estimating its overall effectiveness. Advantage is almost always going to be more effective.
Advantage may be more effective on a single roll, but precision attack will have more effect over the course of an adventuring day. Because the results of a single roll is a small part of the comparison. Because their triggering conditions and frequency of use are so different.
Three rounds of advantage per day, picked before starting any attacks, blocking any bonus action attacks, will have the advantage change a miss to a hit a lot less times then up to four times per short rest (up to 12 times per day?) add in a die only when it can make a difference - not wasted on a roll that would likely hit, or on a roll that still (likely) won't hit.
With a 65% chance to hit, advantage has a 22.75% chance to turn a miss into a hit. (Turning a hit into a hit is no change.) With two attacks per round, so with six attacks over three rounds we can expect 1.365 misses turned into hit. Since it's a bonus action we can't add in bonus action attacks to increase that.
(I had Wolfram Alpha do the math:
6 trials at 22.75% probability of success - Wolfram|Alpha)
With precision attack, assuming we have a good idea of the AC needed, we don't have to waste any on hits. We only have to worry about the 1-7 results that would be a miss.
If we apply them only to 7s, all 12 are good - but that would take ~240 attacks to have enough. With 3-4 rounds * 6 encounters * 2 attacks per round we're looking at say 36-48 attacks a day, so let's aim for more. Heck, let's add it to 2+. That needs roughly 40 attacks to use up all 12.
So the chances are 100% to switch a miss to a hit on a 7, 83.33% (6), 66.67% (5), 50% (4), 33.33% (3), and 16.67% (2), which averages out a 50% chance of success assuming an even distribution of failed numbers (a natural 2 is just as likely as a natural 6).
12 trails at a 50% chance is 6 expected successed. (Wolfram Alpha again:
12 trials at 50% probability of success - Wolfram|Alpha)
So we can see that the number of uses plus the triggering conditions make Precision much more effective than Fighting Spirit when first taken. Both scale, Fighting Spirit with more Extra Attack, Precision Attack with more dice and larger dice. I think the bonus gets skewed even more in favor of Precision later on. Again, the limit on which misses to attempt to turn was based on number of attacks - more attacks mean more potential misses, so you can be more discerning on when to tirgger them because you still expect to use all of them. Since you'd be cutting out the lowest chance, the miss-to-hit change for any particular attack would grow, so you'd end up with more successes in the end.
That can also be realized with a bonus action attack, such as from Polearm Master or Great Weapon Mastery's crit - while advantage would give more crits, you have no bonus actions to take advantage of those since it was used to trigger Fighting Spirit.
And, this is assuming just trying to outdo Advantage in terms of attacks hitting. Being more selective on where to use Precision will up the chance of turning that miss into a hit so there's not a great loss in conversions, but will leave more superiority dice for other uses, such as Riposte.