I have an iPad 3 with Retina display, and there's no lag. The newer models have even more powerful processors, don't they? If you can use an iPad now, buy one. There will always be a better one in the future, and you could wait forever without buying anything.
For me, the question would be iPad 4 or Mini. My wife bought a Mini, because it fits in her purse, and the smaller size is really nice. I'm more active online, though, and appreciate the larger onscreen keyboard of the full-size iPad.
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