I had several packs in Pinball Arcade before they started splitting them up into "seasons", and played it a lot before I found Zen Pinball. Pinball Arcade is great for replaying favorite tables that have been faithfully recreated in the game. Zen Pinball has completely original tables that often times can't exist in the real world. I prefer the physics in Zen Pinball, and after playing both feel like PA is a little "floaty" for lack of a better term. It's like the ball doesn't move fast enough or something.
Anyway, both are good games for pinball fans. I just prefer Zen Pinball because they're original tables. There are only a few tables I really wanted to replay in Pinball Arcade, and after playing those to death I got bored and moved on. There's just so much more that can be programed when the table is completely virtual like in ZP... the limitations of the real mechanical tables in PA just feels boring in comparison.