It doesn't matter what the iPhone's OS is a subset of.
The iPad = success if it stays away from using a full version of any desktop OS. To live on a coffee table and be ready at any commercial break to come alive, the iPad needs to be quick. To play a game or serve up the weather or your email or allow you to browse the web for more info about that car the commercial is featuring, the iPad needs to be quick. It might need to even anticipate your needs and "wake up" on being picked up.
Any desktop OS has too much overhead to be quick:
They have services and processes and drivers and more.
Their hard drive makes noises my ears do abhor.
On boot up we've time for the bacon and eggs.
This boot time would sully that iPad with legs.
I have to conclude Apple's Jobs got it right.
In keeping iPad's basic code nice and tight.
Avoid any tablet with desktop OS.
Remember this mantra:
K.I.S.S.!