After tweaking stats back and forth, I have arrived at the following candidates:
gnome 1
str 8 (-1 attack str, +1 attack size)
dex 12 (+1 range attack dex, +1 range attack size), (+1 AC dex, +1 AC size)
con 19 (+4 HP/lvl, +4 concentration, +4 fortitude)
int 15
wis 15
char 13
human
str 10
dex 12 (+1 range attack dex, +1 range attack Point Blank Shot) (+1 AC dex)
con 17 (+3 HP/lvl, +3 concentration, +3 fortitude)
int 15
wis 15
char 13
halfling
str 8 (-1 attack str, +1 attack size)
dex 14 (+2 range attack dex, +1 range attack size), (+2 AC dex, +1 AC size)
con 17 (+3 HP/lvl, +3 concentration, +3 fortitude)
int 15
wis 15
char 13
All three of the characters listed above have a 15 intelligence. None of them have a net bonus based on strength. Between gnome1 and the human, each has a net bonus of +2 to ranged attack. Since the human is using Point Blank Shot, he will get +1 on damage for ranged attacks (including magical ranged attacks like rays). Gnome1 has +2 AC, compared to the human's +1 AC. I think the AC bonus is more important than 1 extra point of damage on ranged attacks. But what really puts gnome1 over the human is the extra HP/lvl. The halfling has +3 to AC and +3 to ranged attacks. That gives him an advantage over both gnome1 and the human (although he also doesn't get the +1 damage on ranged attacks since he won't have Point Blank Shot). Between the halfling and gnome1, it becomes a question of what is more important, +1 on ranged attacks and +1 AC, or +1 HP/lvl? Halflings have that nifty +1 racial bonus on all saving throws. But gnomes get +1 added to the DC of their illusion spells, which I plan to use a lot.
gnome 2
str 13 (+1 attack str, +1 attack size) (+1 damage on melee attacks)
dex 12 (+1 range attack dex, +1 range attack size), (+1 AC dex, +1 AC size)
con 12 (+1 HP/lvl, +1 concentration, +1 fortitude)
int 17
wis 15
char 13
Gnome2 is different. Unlike the others, he has a net bonus of +2 on melee attacks, and +1 damage on melee attacks. He also has a +2 bonus on ranged attacks and a +2 bonus to AC. He is the only character with a 17 intelligence. That's his biggest strength. His biggest weakness, however, is his low constitution, which will only grant him a bonus of +1 HP/lvl.
At this point I think it's fair to say that I am obsessing over this choice. It's an interesting puzzle. Who should I pick?