They are both different things. One is specifically for rendering games, and XCode is for everything else. I wouldn't buy a low-end Mac Mini for development btw. Think about the processor and how you'll going to constantly make it work at 100%.
I'm a Mac user 100%, and for me Unity3D is nice. I got the 3.0 version on a special freebie deal. Being able to use a plugin that makes coding games easier is really cool, and I love it. I learnt how to put together Pong with Brain Builder in just under a hour and it was playable and I even deployed it to my Android phone.
XCode is very complex compared to Unity. It's strictly code and pretty much nothing else.