They both mean pretty much the same thing. The second form is almost always used, even though the first is technically correct.
There is a subtle twist of meaning here. If I really wanted to compliment a person on being friendly, I would say "You're pretty friendly" or "You're really friendly."
If I say "You seem like a friendly person." the meaning conveyed would be that the compliment is qualified - yes, you seem like a friendly person, but you might not actually be one.
This applies to both the forms above.