Well, it comes down to where you fly.

On United, you tend to get more inflight services, such as audio on almost every flight, and you get your meals on board.

We can *easily* get into a fight over which airline is best.... I've flown both of them, and you can guess which one I am loyal to! United also gives Premier, Premier Exec, and PE 1K members Economy Plus, which gives you 3 to 5" of extra legroom.... delta gives you.... well, heh...

On the other hand, your Delta and United miles can be redeemed and earned on eachother....

again, it all boils down to who you fly the most and whose airline partners suit you more.