First of all the first slender was much harder. The map (or at least from what I saw) was much bigger than Arrival. Also, there were almost no paths, so you had to wander around the woods without ANY sense of orientation... In arrival, the notes were really easy to find, I found 4 in 2 minutes... I still didn't manage to finish the game because it lagged for me (14 FPS) since I recorded the game, but for the graphics/detailing part, arrival totally beats the original slender (although I believe slender isn't about graphics because I'm quite sure everyone will agree that a scary game doesn't necessarily have mega-fancy graphics). I find the new slender fun to play, and it will be updated alot (the creator himself said that), so I'm looking forward to seeing the story behind it
As for which game is better - I prefer the original one.