I highly recommend Ableton Live (
Ableton - Home). Its really simple to use and very easy to pick up everything that it has to offer
i've been using live since version 5 (for about 2 and a half years) and im hooked, i use it for production aswell as live performance alongside cd decks and turntables. Ive just got myself Logic Pro 8 aswell. I would highly recommend that you get yourself logic once you are confident with making tracks etc. and want to get deeper into it as logic is a very comprehensive and complicated piece of software and will take a long time to master all the features
Hope that helps
Reece