If you're just going to play around, download the latest stable SDK for free from http://developer.app.../devcenter/ios/ and install. At the time of writing there was a bit of discomfort with XCode 4. There are numerous advantages including the use of ARC (XCode 4.2+), which is helpful for everyone, even those comfortable with memory management, however there are crashes which can be annoying and disrupt your workflow.
From Apple's site (http://developer.app...hnologies/ios5/):
Automatic Reference Counting (ARC) for Objective-C makes memory management the job of the compiler. By enabling ARC with the new Apple LLVM compiler, you will never need to type retain or release again, dramatically simplifying the development process, while reducing crashes and memory leaks. The compiler has a complete understanding of your objects, and releases each object the instant it is no longer used, so apps run as fast as ever, with predictable, smooth performance.
If you're looking for the most stable environment, then XCode 3 might be the best for you. At the end of the day you'll be happy with either environment if you get used to it.
When comes time to submit your app, you'll need to pay the $99 to be able to submit it to the official App Store. Be sure to test on an actual device before submitting.
Some advice: Make sure you backup your private key that is linked to your certificates (development and distribution). It saves a lot of headaches if you ever have to reinstall. You don't want to have to recreate all of your profiles and certificates.