Both games are classic arcade games that have been remade hundreds of times. I like pong better because there is multiplayer and it basically started the video game industry.
Pong is better, why? Because you could flip out if you miss a shot, other then Tetris where it comes down slowly and you have like, 20 seconds to make a plan (yes, I played pong the other day, I suck at it, but It's waaay better then Tetris).
Both fantastic 'old school' games but as Drace said, pong is too simple and easy. With tetris, you need logic and thinking and it's different every time unlike pong where it's basically the same all the time.