A D-28 and a D-18 are two very different guitars.

The mahogony on the back of the D-18 is going to give you a bit of a brighter sound than what you're going to find out of a D-28, with its rosewood. They both have a particular tone to them and I personally wouldn't even be too apt to compare them normally. The best thing you can do is go and try them out and find out which one you like more. For that kind of money, I wouldn't go near it unless I played it. Even then, I'd buy the exact one that you play and fall in love with. Acoustic guitars are finicky and unpredictable. You'll come across the same model of guitar that all have a different sound to them.

As for my personal choice, I'd end up going with a D-28, but that's only because they offer it with a wider neck on it than they used to now. You can still get it with the 11/16, but a 3/4 neck feels much better to me than an 11/16.
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