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Assuming "all things equal" (retina display, processor, etc.)...
If I were picking the winner based on modern good looks, then the iPhone 4 would be the clear winner. It's sexy and sleek, looks familiar yet very different.
But if plain old durability and/or usability were the deciding factor, I'd have to go with the 3G/3GS design.
The rounded back is clearly more ergonomic, and easier to hold. While the plastic shell certainly looks cheap in comparison to the iP4, it offers much better drop protection as it will scratch/crack/deform without catastrophically disassembling itself if dropped.
Most importantly, the 3G/3GS design protected the edges of the glass screen with that same plastic shell. (this is the REAL reason there were far less reports of screen shatters on the 3G/3GS IMHO)
All this durability concern may be a non-issue for me though, as I have used protective cases and screen protectors since the very first day of owning that good 'ole 3G.