I wouldn't be too hung up on thinness - I find the existing K2 almost too thin already! I ended up buying a nice case to put it in just to make it feel a bit thicker and to protect the screen a little more.
Does the K3 offer that much more than the K2? it should have a slightly better screen but I don't think it's likely to be an unbreakable plastic one. For that matter does the K2 offer that much more than the K1 apart from TTS? Another option might be to buy a refurbished K2 for $139.
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