I think something that people are missing in the CON vs. DEX discussion is the percentage of times the difference actually helps and the fact that we are not generally discussing that much of a difference in bonus between the #2 ability score and the #3 ability score.
For my arcane spellcasters, although I might have a 16 CON, I generally attempt to have a 14 DEX if possible. It's not typically 16 CON vs 10 DEX or vice versa.
So, how much does the extra +1 in the 16 vs. 14 (or 14 vs. 12) case help? Well, 5% of the time, but this can be somewhat misleading.
Getting a higher initiative 5% of the time can at most help 5% of the combats. 95% of the combats, it doesn't help. So here, it is a straight 5%. The Improved Initiative argument is not valid since both characters can have the feat, the one with DEX 14 and the one with DEX 16. Again, it is only 5% of the time where it MIGHT even matter (since sometimes, you want to delay an action as opposed to always going first).
But, having a +1 higher AC tends to result in about 15% less damage (situational dependent). Although it is only a 5% increase in AC, at low level, opponents tend to have about a 33% chance to hit on average, hence, it drops this from 33% to 28% or 15% less damage overall. And, as levels get higher, BAB increases every level (for combatant types) whereas AC generally does not, so this 15% starts dropping down towards the minimal 5%.
Similar "not exactly 5%" differences occur for saving throws vs. damage effects. If you have +5 to your roll vs. a DC 15 save for half damage, you will save 55% of the time for half damage and fail 45% of the time for full damage. Hence, you take 72.5% of the damage on average (45 + 1/2 55). Increasing the +5 to +6 results in taking 70% of the damage on average (40 + 1/2 60). The +1 only results in 2.5% less damage on average (for most characters, characters with Evasion change this back to 5%).
In the 10D6 Fireball case, this means that instead of averaging 25.375 points of damage, the character averages 24.5 points of damage. The +1 to Reflex saves only results in an average of 0.875 points less damage.
Hit points, on the other hand, are increased by approximately 40% (CON 10 to 12), 28% (CON 12 to 14), and 22% (CON 14 to 16) respectively. Discussions of a secondary ability score above 16 generally do not make much sense unless your DM is giving out a boatload of points to purchase stats with. Comparing 22% with the 15-% of combat or the 2.5% of damaging spells illustrates that in the long run, having the extra hit points is generally perferable.
So yes, saving vs. the Fireball 5% of the time results in 17.5 less points of damage in one Fireball in 20. But, having 10 extra hit points (10th level character) for those other 19 out of 20 cases where the result is exactly the same is more helpful since it keeps you alive.