If you're only going to learn one, go Android. Java is reusable in other environments and frankly, it's just easier.
My personal opinion is that Objective C is pretty tedious and annoying. The syntax is ugly and non-intuitive. Again, this is my personal opinion. But having done years of C, C++, C#, I find it bizarre that Objective C syntax is non-obvious. Not that it is particularly complex, but if you know C++, Java and C# seem pretty obvious, whereas Objective C is just very different in syntax.
Finally, Java is platform agnostic. Objective C has few platforms that it's good for and you have to buy Apple hardware to build iPhone apps which to me is plain stupid and I think in the long run, it's going to be one of the things to hurt the iPhone.
Just my own opinions based on my experience with both. I sat down and immediately started writing Android apps using the SDK and simulator with no previous Java experience. Even after several days of playing with existing iPhone apps, I had difficulty even following what was happening in the code, understanding the stuff I was seeing in the watch windows, and figuring out exactly what the various syntactical crap meant.