Tooth of Chiss-Goria costs three mana, has Affinity for Artifacts, can be played as an instant, and can tap to increase target creature's power by one until end of turn. Scale of Chiss-Goria is the exact same card except that it adds a point of toughness instead of power. Now, you would think that would be no debate about which of these two cards is the better one to have in your deck. Any reasonable player knows that power is better than toughness, right? Consider Alpha Myr and Omega Myr. The former gets at least some amount of respect and is certainly at least considered to be playable. Not so for Omega Myr, the downtrodden brother of the pair, and the one with the extra point of toughness.
Well, I'm here to tell you that there are quite a few pro players who think that the Scale is better than the Tooth. In fact, I'd go so far as to say that in my lifetime, the number of players who have answered the above question by saying "Scale" is roughly equal to the number who have answered "Tooth." Furthermore, there are Tooth people and there are Scale people, but there's absolutely nothing in between.
So what are the arguments?
First of all, Tooth proponents cite the fact that once the cards are on the table, the Tooth will simply have a much larger effect on the game than a Scale would. Every time your opponent lets a creature through, they're taking an extra point of damage. Scale, on the other hand, doesn't actually put any pressure on your opponent, and rarely has much of a lasting effect. Another convincing argument, and the only one for Richard Hoaen, is the existence of Spikeshot Goblin in this format. The difference between pinging for one and for two is extremely pronounced; so much so that a Spikeshot backed with a Tooth is generally a dominating force, whereas a naked Spikeshot is only an annoyance. Finally, the surprise factor on Tooth, while admittedly worse than that of the Scale, will still sometimes let you kill a creature that you wouldn't have otherwise.
Fans of the Scale, on the other hand, are generally fixated on the fact that the surprise value of the Scale will often allow you to save a creature. Gerard Fabiano cited two specific circumstances: your Myr Enforcer being targeted with Electrostatic Bolt, or your Somber Hoverguard entering combat with a Skyhunter Patrol. Patrick Mello simply said that he's a more controlling player and he prefers to keep his creatures alive and kill his opponents that way.
Mike Turian, a Tooth player to the core, had this to say: "The thing about Scale people is that it doesn't matter how much they play; they'll always be bad at Magic."
Who would have thought that these two barely playable cards would spark these kinds of discussions? Another interesting anecdote is that the Chiss-Goria pieces have been rising in value recently due to the respect that another new contender is currently receiving. Fractured Loyalty, a card that many initially dismissed as unwieldy and unreliable, has now been finding its way into successful decks in multiples because the effect of controlling any enemy creature is too strong to be ignored. Both Tooth and Scale are strong cards in this sort of deck because they provide repeatable targeting, often completely for free.
Anyway, next time you're at your local card shop, ask the other players, "Tooth or Scale?" and see what they have to say. The discussions it provokes can be quite interesting.