No armor pen on avg high armor level 1 target
52.14 (60.05) damage (35 armor)

/w armor pen on avg high armor level 1 target
50.46 (59.16) damage (35 armor)

(with Doran's Blade in parenthesis)
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% Reduction

60.10532
61.35415

55.28927
56.85191

49.15081
51.16233

41.05858
43.74437

29.90326
33.66940

13.53968
19.19198		 Twitch



300
300

250
250

200
200

150
150

100
100

50
50

^Armor		 100

39.89
41.93

44.71
46.82

50.85
52.99

58.94
61.04

70.10
71.97

86.46
87.68		 Twitch 150

59.84
61.25

67.06
68.38

76.27
77.41

88.41
89.17

105.15
105.13

129.69
128.08		 Twitch 200

79.79
80.58

89.42
89.96

101.70
101.83

117.88
117.29

140.19
138.30

172.92
168.48		 Twitch 250

99.74
99.90

111.78
111.54

127.12
126.25

147.35
145.42

175.24
171.46

216.15
208.89		 Twitch 300 <--- Damage

119.68
119.22

134.13
133.11

152.55
150.66

176.82
173.55

210.29
204.63

259.38
249.29
Conclusion

No armor pen marks does 2 more damage early 4 less late
Armor pen marks does 2 less damage early 4 more damage late

8.5 damage is roughly equal to 15 armor pen at red and blue buff for Twitch Jungle.
8.5 damage is superior to 15 armor pen at all other creeps for Twitch Jungle.

Last Whisper scales better with AD marks
Black Cleaver scales better with ArP marks

With just one armor item Last Whisper becomes superior to Black Cleaver average damage
without even factoring +armor spells (may need retesting)