I was in a very similar position to you having a mac desktop, ipad and having had an iphone 4 (and other various iphones since the original in 2007). I got a great deal on a refurbished S3 in November so I took the plunge.
I will be honest with you, the first month or so wasn't easy. Until I learned the ins and outs (somewhat) of Android I had some frustrating times. I consider myself fairly intermediate when it comes to tech stuff and it took me a bit to get the phone the way I like it, and it's an ongoing evolution.
Lately everything's been working great. Partially because of this forum and being able to come here for advice and read the experiences of others. Now I love my phone and when I pick up and play with an iPhone I immediately miss many of the features of my SIII. Iphones are pretty and generally easier for novice users to get up and running, but the experience is like a walled garden unless you jailbreak.
I also echo a previous poster who urged you to wait and see. One thing I love about the android environment is I don't have to feel like a slave to one OS, carrier or manufacturer. A lot of great new devices will be coming soon this year, including very likely a galaxy sIV if the rumors are true.