What's the difference between "pidän" and "tykkään" ?
I would like to know if there's a slightly different connotation, maybe one expresses more love than the other or is less awful when expressing such feelings in person.

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Tykkään sounds more casual, colloquial, informal... and puhekieli-ish! Pidän is used in puhekieli (which refers to spoken Finnish) as well all the time though!
On the contrary like I already implied in the beginning, the verb tykätä isn't part of kirjakieli, or written Finnish. I don't think there are any further nuance differences!

Wait, or do I? Because of this very slight one nuance difference, saying "tykkään susta" instead of "pidän susta" might sound little more adorable/cute, because I do think tykätä is used more by e.g. children who talk about other kids they have crushes on. But the nuance is, I think, so little that there might be other Finnish-speaking Finns here that could even disagree with me.

The verb pitää has maaaaaaany other meanings as well, such as to wear, to consider/to think something as something, to hold (onto)/to grasp, should do/must do... So please bear this in mind.

You might already know this, I don't know, but when expressing the state of someone liking something, you have to decline the noun that the verb pitää is referring to to the elative case. :) It's the one that ends in -sta or -stä. When pitää is used in those aforementioned other ways, other noun cases are being used. :)

AND the elative case is also the noun case that's used with tykätä as well.