Both are good, though Zen still doesn't support iPhone 5 resolution... but that's not a concern for you. If you have a Mini though, I read performance can be iffy in The Pinball Arcade, but it's possible that's been fixed in an update (they come out with new tables every month in TPA).
But why pick? Both are free to start with, and they are not too similar. Zen is fantasy tables, and TPA is all simulations of real tables. Price is the same roughly speaking over both, and there's no real reason to go with one or the other unless you have a preference for a certain style yourself. If you want to see what it's like playing some of the classic pinball machines like Twilight Zone or Star Trek: The Next Generation, well Zen's not going to do that for you, and if you want to play the fancy soon-to-be released Star Wars tables with 3d characters running over the screen and light saber battles, you won't find it in Pinball Arcade.
I personally think The Pinball Arcade is the stronger title, it has more consistent support, and I prefer playing the recreations of real tables, but both are among the best choices for iOS. Zen's missing quite a bit of tables compared to the PS3/360 counterparts though, iOS only has maybe 1/2 the Zen tables released.