I think I have to go with Constitution, but it's a tough call between that and Dexterity. I'd go with Intelligence as #3 and Strength as #4.
Both CON and DEX give a bonus to a save, so that is mostly a wash unless your DM throws a lot of Evocation spells at you.
But, to me, the extra hit points are extremely important.
10th level opponent throws 10D6 Fireball at your 10th level Sorcerer or Wizard.
That's 35 points of damage against him where his average hit points at level 10 is 26.5 (i.e. 4 hits at level one) with a CON of 10.
If he misses the save, he is at -8 or -9 on average and about to die.
Having the 14 DEX will enable him to make the saving throw 10% more often, but that does not help a lot on those times when he rolls low on the save.
In other words, the bad guys will throw stuff at you often enough that sooner or later, you'll miss a save on something that does hit point damage.
With the extra CON (say for example 14 where you would have 20 more hit points in the example above), you can at least survive that. Without it, you might not.
With the extra DEX, you will take full damage less often (5% to 15% less often depending on the differences between your DEX and CON), but sooner or later you will still get smacked hard.
To me, surviving is more a matter of having options and continuing on than it is a matter of having that extra +1 or +2 to hit with a ranged touch attack that may or may not work (and the +1 or +2 will rarely matter depending on the roll), or the bonus to AC. Even at one hit point, my Wizards can usually attempt something. At -1 hit points, they cannot.
But then again, I always create defensive Wizards as opposed to offensive ones, so YMMV.