You cant focus on a bus anyway. So go for the ipad its awesomeSimilar situation I'm in: commuting each day one way 40 min by bus.
I can sit down so I put my MBA on the knees and use the keyboard. If you can't sit because the bus is crowded the iPad might be easier (or even iPhone)
So I have all three: iPhone for music, iPad for reading and surfing, MBA for my development in Xcode for iPad. Anyway: development on iPad would be difficult so I "need" both. Ok, not really need but for sure "want".
I tried lots typing on iPad in bus but it was a bit too shaky and the screen too small. A BT-keyboard might've help but the again the MBA woul be better.
If you get MBA: take 4GB; you can't upgrade later.
You cant focus on a bus anyway. So go for the ipad its awesome
Since you will use this while in transit, I think an ipad is a better fit for you.