It used to be that there wasn't much difference between the Apple iOS (for iPhone and iPad) and the Google Android OS (various phones and tablets) - they looked similar and they had a pretty similar set of apps. When I had an iPhone the apps I used the most were Google Maps and YouTube. Recently Apple decided to use their own mapping program and now they have announced that YouTube will be removed on upcoming iOS updates. Both of these moves seem like a huge mistake to me since Google Maps and YouTube are pretty much the standard in their segments, but Apple and Google have never been friends. Steve Jobs famously said,

"I will spend my last dying breath if I need to, and I will spend every penny of Apple's $40 billion in the bank, to right this wrong," Jobs told his biographer. "I'm going to destroy Android, because it's a stolen product. I'm willing to go thermonuclear war on this."

If you are looking at getting a phone or tablet you have to examine your usage - are you Google person or an Apple person? What are you using your device for?
- do you depend on Google services like Gmail, Google Maps, Picasa? Choose Android
- are you an iTunes addict and need your iTunes with you at all times? Choose Apple

Personally I never got into the iTunes thing and I find that Google has a lot more to offer in way of their own apps, but that's me. Just make sure you take a close look at what you plan on doing with your mobile device before you choose an OS.