I've found that sprites and 3D plains are about the same speed. My last game, I used sprites for everything, and found the ease of them very handy - like changing UV coordinates instead of changing images, that sort of thing.
My current game uses plains, and the zdepth issues are a real headache, but if you can get around that, then it is very nice to have that resolution independance.
Most netbooks have a 1024x600 resolution, but really we should consider the next stage in netbook evolution - 1366x768 for example is what my netbook runs at. So I'm trying to go for 3D plains, with over-resolution graphics. If I was using sprites, then I couldn't do that easily, not like my last game which had a backdrop and front - so the backdrop could be any size, and the front would be pixel perfect.
Really, I think it depends on the game - some games are easier to do in pure 2D with sprites, and some games make sense to do in 3D. If you have scrolling maps then I'd say go for 3D plains - or any type of tiled map would be much easier in 3D.
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