Not much of a guide; you're really just giving a lengthy tooltip for those two masteries here.

7 lethality=/=7 armor pen, it's more like 2 early on. You're probably getting a 2-4% damage increase from it until you supplement lethality with items and higher levels later on.

Early on, cheap shot gives comparable damage to 7 lethality (cheap shot is marginally stronger), but you can proc it more than once in a skirmish. Cheap shot is also easier to access than sudden impact, because sudden impact is contingent on using Q. It also bypasses armor buffing abilities, for what it counts.

Cheap shot is stronger than sudden impact specifically early game in the actual jungle part of the map. It's not stronger for ganks, and sudden impact outscales it. It is strong for jungle control and counter jungling (when you want to hold on to Q).

Cheap shot is an easier, safer, more accessible rune than sudden impact, and I certainly wouldn't discourage using it, especially for less experienced players. It has its place.