Damage:
3d6+2: 3.5 * 3 + 2 = 12.5 average, 20 max
1d8+7: 4.5 + 7 = 11.5, 15 max
Chance to hit:
+4 vs 15: 11 or higher will hit, 20 will crit (45% chance to hit, 5% crit chance)
+9 vs 15: 6 or higher will hit, 20 will crit (70% chance to hit, 5% crit chance)
0.45 * 12.5 + 0.05 * 20 = 6.625
0.70...