I could only test this with Air Shield and Air Armor, as I can't buy any other spells due to the inventory full bug, but from some initial testing:
Code:
Air Shield Air Armor
16 11
15 10
17 11
14 15
14 11
8 17
12 10
12 16
9 10
12 14
19 6
16 9
13 15
10 11
16 6
11 18
17 15
13 14
10 18
14 6
12 12
13 8
17 11
12 17
8 13
15 14
16 5
11 15
15 10
10 7
AVG 13.23 11.83
MIN 8.00 5.00
MAX 19.00 18.00
===
30 casts of each spell at the same skill level of 14
Since Air Armor is a higher level spell, than Air Sheild, I was expecting it to offer better protection, but looking at the numbers, Air Shield wins out overall.
-p