If you're due for an upgrade, I'd recommend waiting a bit because the S4 should be arriving soon and its essentially a more powerful S3, design language and everything is the exact same. Now, will it be laggy? I've seen S3s lag so it depends. It could be all the clutter that comes stock in touchwiz home screens that slows it down when flipping through your home screen. Other than that, not much lag.
But what you should understand is this is NOT an iPhone. One of the main complaints I see from ppl who switch OS is that "X feature isn't on the S3 and I loved it on the iPhone, what's the closest thing to it?" Its too much to find and replace each feature from the iPhone. Doing that, u would be better served just keeping it. Try to enjoy the Android/Touch wiz exclusive features and get use to those. We don't have something like iMessage built in. Get use to that. We DO have tons of useful features though. Be open to exploring it.
And as for ease of use, I think Samsung has one of the simpler interfaces around outside of the Nexus line, so I'd reckon itd be easy to get use to. Don't be shy to look into the HTC line of phones either though. The One looks like its simplified things a bit more than Samsung has. Worth checking out.
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