The S3 is on sale today at Best Buy, $50 with contract for any carrier. Sounds like if storage is important to you, that's the way to go. But I personally think the N4 is the best Android phone out there.
You're brave to try the switch. I had the iPhone 3G and loved it. At the time, it was the best phone out there. But I wanted more control, and when the Nexus One came out I jumped ship as soon as I could (even paid an ETF to get out of my contract with AT&T). I have never looked back, and I think the Android platform is far superior (accepting the fact that games are better on the iOS platform, but I don't play mobile games). However, if I was in the iOS environment for as long as you, I probably would have a hard time switching. Good luck.