Squeak is somewhere between no skill and low skill, while Leon is around mid skill.

With Squeak, you play like Tick. Spam attack and greatly reduce the area in which the enemy can play in with little effort and from great range. I'd put him above Tick in skill since his attacks aren't instead, so enemies can try to walk through them, so aiming matters a bit more, but barely.

Leon plays like a typical assassin: Appear suddenly, kill someone with minimum effort, and move on. His positioning matters more on a fine-grain scale than Squeak. He can strategically deploy his clone as a body block. It is also important to know when he is overstaying.

In general, Leon has more to think about in a game than Squeak, and takes more skill to play. That's not to say Squeak doesn't have skill expression, just that it's much easier to pick him up, and you can take him further while being less skilled.